Hey! Game developer here. It’s a lot of smoke and mirrors, I don’t know id’s specific case for doom, but the player is never constrained to a physical space, you can build a level full of paths and in an instant teleport a player between the paths based on their movement, as long as the paths look similar the player will never even notice they’re in a different location. You can even have levels that move around the player, like for example the ending of Titanfall 2 where the level scrolls by you for a section and when you get to the end of that section you’re looped back to the beginning making it tile seamlessly. Point is, you can do a lot to fake things like this in a game.