Ownership is just a social construction.
Social constructions can be important.
Very small kids have a really hard time understanding ownership / sharing / borrowing. When they want something, they just want it, and cry if they can't have it or it gets taken away from them.
So, if you're the only one with keys to bitcoin, you own that bitcoin. Nobody else can feasably own that bitcoin without getting the key from you.
Authority decides who owns what.
When the biggest guy in the room says "that's my seat", that's the biggest guy in the room's seat. When somebody has a gun and says "that's my seat", it's the person with the gun's seat.
The authority on bitcoin is the ledger. Rather than something centralised like a government body or court.
Bitcoin doesn't decide who owns what, it is just the enforcement of mathematical rules and we can derive ownership from them.
