If the DRM is enabled, the game does a simple "Is the game available in the user's library?" and steams says yes or no.
If the game didn't have DRM enabled, no check is made. Copy the game folder elsewhere, without steam install and it should launch.
Devs can enable the DRM afterward, but your copy won't be locked.
But even then, if valve goes bad guy, the DRM is simple enough to be broken, and there is no double check or something preventing you from playing (unlike Denuvo which encrypts the game and has multiple separate checks for the DRM).
Yes (that's the point of a DRM), but like I said, the DRM is easily broken. Some games can also still use steam features when cracked (like joining lobbies, inviting friends, etc), and it's the same "crack" for every game (not withstanding other DRM the game may have).
With Valve, I'm more concerned of not being able to download the games if they go under, than the DRM on the games I have. Over time, the Steam DRM has also been more permissive than before, as I can now play my "family's" games and they can play mine.
Part of the apparently forgotten but huge amount of work that went into making digital storefront for games that people trust to work was that Valve publicly talked about verifying things such as a procedure to globally strip DRM from all games, in case Steam was to cease operations.
In the first case they can just refuse to let you use your copy when you ask for permission.