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.