That’s a basic rundown. I’ve mostly learned through trial and error. You can find the files in dir/data/json/mapgen. The best way to learn is to try.
You can force-spawn buildings by going in the debug menu, pressing ‘g’, then pressing ‘o’ and using ‘/’ to search for the name of what you’re trying to spawn. You can also view it in a natural way by using debug to reveal the map and then teleporting to the location.
Unfortunately there isn’t a lot of ‘odd’ meat in the game. I don’t know the rules on adding items, I’ve just been doing a little mapgen stuff, working with things that already exist. Hence what I said about the lack of human organs. I could create those items, no problem, but plugging them in to the harvest code and all that jazz is beyond me, so I defer until someone else does it.
I also had not seen a butcher shop in a long time. In testing, however, they seem to spawn an awful lot.