[aasasd]

Fun fact: afaik many games, starting with Duke Nukem 3D, implement mirrors by just having the entire room built out behind the glass, with duplicates of the objects moving on that side. In DN, with some noclip trickery or something, one can move into the ‘mirror room’, or observe the whole level with the double rooms.

[Agentlien]

Yes, that's why you need to make such a room when making a map with a mirror in Duke Nukem 3D.

Modern games also do a lot of tricks when it comes to mirrors. Real reflections are expensive. Though they're becoming increasingly common thanks to the ray tracing hardware on recent graphics cards.

As a player I recently noticed that in The Last of Us Part 2, when you're in front of a mirror the windows give off an unnaturally bright light to hide that the outdoors area wasn't reflected.