This is a copy of the Backrooms. Tthulhu decided to copy this idea for his simulation. So there's infinite realities of this place located behind Simulation 3.0. To enter the place, noclip through a bunch of rooms. As each door is opened, the place gets darker. There are flashlights though, but you need batteries to keep going. They are in random places. There are two enemies named A-50 and A-200. They eat your soul and to survive when they chase you, you must hide in a locker or refrigerator.
As seen in the image it shows the room is named A-001. But A-999 isn't the end. A-99999 is the end... for letter A. Then comes B-001. The process repeats over and over and the final room is Z-99999.
The room shown in this image is the starting base. The door Luigi is looking at opens once someone passes door A-50. The enemy A-50 comes out of that open door, and gets ready to chase now. Later after passing A-200, A-200 then comes out. The starting door also ends up leading to an infinite hallway. Pass infinity there and you reach the Dreamrooms.