how this game made millions of rooms using 1 polygon

lately I've been playing a bunch of games that take place in large sprawling cities like Spider-Man and cyberpunk 2077 but there is one thing that's been bothering me the buildings when you look at a game like Spider-Man every single room visible within the buildings has depth and is fully furnished this blew my mind so I decided to learn how this technique works and as it turns out every single room in these buildings is completely faked using a ve...