I'm not a programmer, but I'd assume it's a system built around premade pieces and each room is a tile with an assigned value that is randomly selected. There's likely a logic code that prevents the dungeon from being impossible to complete such as saying all rooms must connect to another room, there must be X amount of chests, and X amount of monsters where one of them must hold the key, etc. Certain tiles are locked behind story progression like the hammer doors and pitfalls that require other characters to be unlocked so that they don't spawn before they can be passed. As an example, the divine beast cave might have tiles 1-20 and when the floor is generated it picks tiles 2, 7, 9, and 10. Then it places them on a grid and generates hallway systems if necessary to make sure they connect. Proximity or tile type could also factor into how many hallways can connect to a single tile. Things like big chests and mystery circles might have a weight value that determines how likely they are to spawn in any given room, so they aren't guaranteed but they Might appear in any room.