glad you liked the art!
I'm thinking about writing a blog post describing the level generation in detail, but the basics are:
1. use multiple passes of Perlin noise to generate the floor decoration
2. randomly generate the overall dungeon layout using the method described in this video. There's some logic to be able to set a max "depth" (basically number of rooms), and to make sure there are no doors going to nowhere.
3. Just like you guessed, fill out each room from a list of pre-made levels. Again certain layouts only work for certain doors.
if you're not afraid of some lisp, here's the world generation code https://git.sr.ht/~alect/fantasy-console-carts/tree/master/item/snowfort-siege.f...
hope that helps :)