LadyWYT

First off, welcome to the forums!
I've heard that boss health is supposed to scale in comparison to nearby player count, but I'm not 100% sure that it does. Overall, I would say that story locations in general are a little easier with a friend or two, simply because you can watch each other's backs or revive each other in the event someone falls(thanks new healing mechanics!). Very minor spoilers in that so far there is one boss fight per chapter, but not every story location contains a boss battle.
 
Each story chapter has at least one "challenge" portion that will prove quite difficult on average, though how difficult it actually is depends heavily on individual player skill. I highly recommend taking temporal gears with you to reset your spawn, in the event that you meet with misfortune.
 
Again, it depends on player skill. On average, the main puzzle locations take a couple of in-game days to complete, minimum. If you've never done those locations before though, I would prepare to spend more time there overall, as some puzzles can be quite challenging.
 
The answer to this one really depends on whether you prefer to complete content solo, or with friends. Completing it solo, you can take as much time as you'd like at each location, and won't need to worry about sharing loot. If you decide to complete it with friends though, hilarious hijinks can ensue, and overall your reactions can play off each other.

Heh heh heh...this is a certain story location from chapter two, and will appear in all worlds that have been updated to 1.20 or later, assuming that lore content is enabled. If you are not currently pursuing the main story, you can venture closer to take a peek at what it is, but I don't recommend venturing into the location itself until you actually have the mission to go there.
Spoilers:
 

Welcome to the forums! 
My initial questions are which mods, and does the issue still happen in the vanilla game? Not saying that a mod is responsible, but when playing a modded client mods are generally the first thing to check when issues arise.
Otherwise, I would suspect that you might be playing with VSync turned on. There were changes to the rendering system in 1.21, I believe, that seem to be more noticeable on certain hardware with VSync enabled(ie, produces a graph like the one in the video). Turning off VSync may smooth out the graph, however, I would also note that your framerate still seems to remain high despite the issues. Unless your frames are dropping enough to make the game actively choppy to play, I'm not sure that I would worry too much about it, in the event it is VSync related or something.

It used to be a thing, but was phased out in 1.20 I believe. You'd have to mod it back into the game.

To my knowledge they can mature, however, unless you're intending to keep them as curiosities, there's not really a point to keeping them around. They can't be domesticated, unless you're using mods to do so, and the resources they offer aren't very good in comparison to other animals, especially proper livestock. 

Welcome to the forums! I am pretty sure that if you placed torches at point A and point B, you'd get lighting like the very bottom strip. That is, both ends are the brightest spots, with the light getting dimmer towards the middle.

They are, but unfortunately, that is likely the cause of the problem. Herbarium received an update recently, but I'm not sure that the issue is fully fixed. The Wildcraft mods themselves aren't yet updated for 1.21, save for the trees module, I think. There is an unofficial mod that makes the Wildcraft mods function on 1.21, but that's really about all it does...makes the mods run. It doesn't really fix any underlying issues that might exist.
In any case, you can try checking for any updates of the mods you're using, and seeing if installing any applicable updates fixes the problem. Otherwise, I would recommend starting over and leaving out the mods that are causing problems.

Welcome to the forums! If you change world generation settings, the new settings will apply to newly discovered chunk, but not the old. You would need to regenerate old chunks entirely for the changes to take effect. 
To regenerate chunks, I think the command is /wgen regen [chunk radius]; I do not recommend doing this in chunks that you've built in, or chunks that otherwise have things you wish to keep. If you go this route, I also recommend keeping the chunk radius value small, as the more chunks you try to regen at once the longer it will take.
Of course, if you aren't particularly attached to your current world, there's always the option of starting anew with the correct settings. That saves you the hassle of trying to fix things via commands.

Nah, you can just wait until the pile burns out before refilling--it won't hurt anything. The process will just remain paused until you refill and reignite the fuel piles.

If you aren't having any luck in that area, I would try somewhere else. Ideally, you want a High or better reading, but it also doesn't hurt to try digging at Poor or Very Poor readings. Since iron deposits are so large, they're generally hard to miss when digging a mineshaft, so if you dug a shaft and found nothing best to try elsewhere.

Welcome to the forums! This is what you're looking for: 
This is the actual guy behind that track in Vintage Story as well; it might be one of the only music pieces currently that wasn't made by Lo-Phi.

Are you using a watering can specifically, or just a bucket? To my knowledge, you need to use the watering can on them, as other vessels won't work.
 
Should be as simple as placing anything else on the ground: shift + right-click.

Welcome to the forums! My first question is...are you using mods, or is this happening in the vanilla game? And if vanilla, then which version?

First off, welcome to the forums!
Rivers are not currently a thing in Vintage Story, unless you are using mods. There are oceans, but how common they are depends heavily on world settings. By default, large bodies of water aren't common at all, but you will find an ocean eventually if you explore enough. For a better chance at finding oceans though, it's best to reduce Landcover to something like 80%; doing so ensures a world that is still primarily land-based, but has several seas/oceans to explore.
As for rock types...it varies by seed. Generally, most worlds will have a selection of key resources near spawn, but require you to explore for others. Sometimes you get a spawn that has everything you need right by spawn, but that's fairly rare. And sometimes the spawn is lacking in resources so the deck is stacked against you. If you're not impressed with the spawn of a world, best to create different worlds until you find one that's fun for you. Aside from that, the idea is to just roll with the punches and make the best of what the map has to offer.

Welcome to the forums! Typically January is the coldest month, at least in my experience. If you're wanting to settle somewhere warm enough for crops to grow year-round, the easiest way to accomplish that(without starting a new world) is to trek south until you find redwood or bald cypress. While not tropical areas, the areas should still be warm enough to grow crops year-round, especially if you have greenhouses.

Judging from the map, it looks like it has spawned. I recommend checking out the nearby ravine.

Nope, that's just how firepits work in general. Once it's done cooking an item, the temperature will reset for the next item when it starts cooking.
It's also why this mod is so popular: https://mods.vintagestory.at/betterfirepit

Welcome to Vintage Story!
Torch holders keep torches placed within them from being extinguished, whether it be by rain, wind, or simple burn time. They're a better option than oil lamps since they offer better light, but are easily outclassed by lanterns(cheaper and more light).

Welcome to the forums! The vault should be on your right, if you're looking at the eidolon. If you visited Tobias prior to 1.21 though, you may need to regenerate the location in order for the vault to actually be there.

The little greenish blue particles are phytoplankton; you'll find them in areas that are sufficiently warm.

Like @williams_482 noted, those trees are further to the south on default starts. Redwood you shouldn't have to travel far for, as it's in the warm climate band and easy enough to find with a day or two of traveling south. You can also buy redwood seeds from agriculture merchants, if you'd like to skip the travel. Also found in the warm climate are bald cypress trees, which are found in swampy areas specifically.
Ebony is in the tropical zone, which will take several days of traveling south to reach. It tends to be found in drier areas, much like acacia, and the two look similar but ebony trees are much bigger. Purpleheart is only found in the jungles, same as kapok, but unlike kapok is quite rare. They don't drop very many seeds at all, so if you want to replant it make sure to shear all the leaves first. Kapok and acacia seeds can both be purchased from agriculture traders as well, but ebony and purpleheart you will need to physically find.
Larch trees you can find in colder climates; the best place to look for them is on top of mountains or climates of the far north.

Welcome to Vintage Story! To my knowledge, hares cannot be domesticated(currently), so they aren't affected by generation.
For a pet hare, you might try one of these mods, although bear in mind they are outdated so they may or may not function correctly. I would test them first before using them on a more permanent world.
Cuniculture: https://mods.vintagestory.at/show/mod/22231
Cuniculture - Rabbit/Hare Farming: https://mods.vintagestory.at/cuniculture
Pet Hare: https://mods.vintagestory.at/harepet

Yeah, can confirm that wild crops will mature, and keep cycling through growth stages after--I've seen mature crops revert to seedlings before my very eyes too(right before harvest, doh!). It is, however, a very slow process, if you're after seeds best to just pick all the crops in the patch instead of waiting on them to mature.
As others have noted, crops don't grow unless it's above 0 C; otherwise they are dormant. If the weather gets too cold, the crops will take cold damage and eventually die. I will note that only applies to cultivated crops--wild crops are unaffected.
For grass, the grass plant itself won't grow back in winter if harvested. New grass will spawn on fallow farmland though, and grass will grow on bare dirt blocks, so that's likely what's happening here. And as @Professor Dragon noted, the scythe has two different modes for cutting grass: the harvest mode will harvest the grass but leave the plant intact so it can grow back later, while the cut mode will destroy the base plant so the grass won't grow back.

I've never seen higher than Decent for Chromite. I would say dig a little deeper on the current shaft(like, close to the mantle deep), dig a few branches around just to make sure you didn't miss anything, and then look for another prospecting point if you still find no chromite vein in that shaft. Chances are that if you went through all the above and found nothing, then the ore didn't actually spawn.
Chromite is also one of those minerals that's rare enough, that I'd say it's worth checking the Poor/Very Poor readings too, if you've got no other Decent reading spots to search.

Absolutely.
 
1.21 has a stable release now, which should be fine for starting a new world without major issues. It should also be fine to update existing worlds to 1.21, but you might want to make a backup just in case. For updating a pre-existing world to 1.21, @Professor Dragon gave a great rundown on how to do so here: 
If you created a world in a pre-release or release candidate of 1.21, that world is most likely no longer viable, and you should start a new world on the stable 1.21 release. You could always try playing one of these worlds anyway, but keep in mind that it's likely there have been major changes/bug fixes since the unstable releases. Starting a new world on a stable release will ensure that everything has the best chance of generating and working properly.