Is it possible to let someone else use your house? Like I gave someone the code but they can't unlock it. what about the garage?

nope, not for houses, nesteds, garages, safes...

but gates and locked vehicles can be unlocked by anybody who knows the code

and strongholds got several owners, kind of...

Speak with the serveradmin. In originsguard is a funktion to overwriting all buildings to another player-id.

In all technicalities if you or a friend were an admin you could assign the house to a friend and switch it back to you. But that would require stopping and restarting the server every time they wanted to get something out. I'm also not very familiar with this feature in origins guard as i have never used it and i don't know how how it would effect a player who has his own building of the same type.