the first one is just a regular player-sized hole in the wall's collision, followed by a seamless teleport into an identical but 10x scaled up version of the same level haha (told you it's all seamless teleports!)
the hallway was really the only complicated part programming wise, but it's just cut up into pieces that're added and removed on demand just out of view, with each piece having 2 doors that spawn a random room when opened. there's nothing behind any of the doors until the moment you open them, and as soon as a hallway piece is out of view it and its attached rooms get removed from the world, but saved in memory so you can return to it and it'll be in the same state and place as it was when you left it.
the way they can occupy the same physical space without clipping into eachother is because of a silly convoluted and probably abysmally slow shader i wrote, which just discards any pixel that isn't covered by an imaginary rectangle placed inside that room's doorway (hope that makes sense). also, only the room you're currently inside actually has its collision enabled, so that solves the problem of bumping into invisible objects from other rooms.
if you're curious and brave enough, the full project files are public, but i'd strongly advise against using it as an actual learning resource since it's a horrific mixture of me never having made a 3d game (or anything more than a simple single-screen prototype) before and obviously this being a game jam so i just hacked everything together as fast as possible.