Pre-gelatin is used to make raw gelatin, then raw gelatin is used to make either plain or flavoured gelatin! I wrote out some notes on it because I honestly haven't been able to commit it to memory, it's a lot of steps from start to finish.
1. 1L bone broth + 1 bone = pre-gelatin
2. 0.1 pre-gelatin + 1L water + 1 bone = raw gelatin
3. 1 raw gelatin + 1L water = plain gelatin OR 1 raw gelatin + 1L water + 1L water + 0.1 syrup = flavoured gelatin
For flavoured gelatin, you need two spaces of 1L water for the recipe to work - which is why I wrote it like I did. I never wrote down how much you get for one step of the recipe because I always do it in bulk at random values. Also worth noting, I've read that you can sub in juice or sauce for the syrup, but I've never actually done it that way myself so I didn't include it.
I mostly use gelatin to make fruit bars, but you should be able to use it in a bunch of different things. Pies immediately come to mind, but I can't remember the full list.