Pyxishroom

I've been playing around in creative to help plan my survival world buildings and made something I really like. It's a little complex and I remember reading somewhere that there was a schematic mechanic built into the game. I cannot, however, find where I read that or any mention of something like it on the wiki. Did I just dream it up or is this an actual thing? And if it is real where can I find a tutorial/explanation on how to use it?

I've been playing around in creative and made a really beautiful slab to cover water sources in my greenhouse. When I went to put it in the water source though, I realized I had chiseled a bottom half slab and it did nothing to cover the water source. Is there a way to convert this into a top half slab or is it just stuck like that? I'd really love to not have to rechisel the entire thing again. (Picture of failed slab cause I am actually really proud of it)