He keeps it for his trophy case/collection. When you see a title change hands, for tv purposes, the belt is handed to the winner but after they return to the locker room the belt is returned to the rightful owner.
when a fighter wins a title is it his to keep or does he have to defend it a certain amount of times before keepig it???
When the fighter wins, the title belongs to him.
As part of the agreement of holding the title though, he must defend it.
Against the next best fighters who are rated along the leaderboard, usually.