It works in the same way that dumping your GameCube games and running them on Dolphin works… It’s quick and easy, but it’s against the ToS and requires breaking DRM.

Steam’s DRM is weak, and in some interviews some Valve developers even gave hints that this is on purpose. Many Steam games will simply run without Steam if you just double click the .exe in the install folder, and the vast majority that only rely on Steam’s DRM can be opened by running a free “Steam Emulator” software that pretends to be an active Steam account with a correct license.