Cut a hole in your actual floor and look down a shaft. Or stand a doorway in the room and walk through it — the world on the other side closes around you, and your real room is what you can see back through the door.
No stencil buffer, which is the usual way and the one thing about this that might not have been available. Instead a large invisible plane with a hole in it is drawn depth-only before anything else. It writes no colour, so your real room shows through it — but it does write depth, so the world on the other side is rejected everywhere except through the hole. Same result, and it needs nothing this phone has not already proven it can do.