Do all the clients send a transformation matrix for each of the hit boxes which make up the character?
The server is running the game as well. And because it's transmitting your character's transform to every other client, it knows where you are. It can work out your character's hit boxes in exactly the same way that the client does.
Sending the hit boxes' location up to the server would be a recipe for exploits. There'd be an extremely trivial hack that simply makes you invulnerable, if that was the chosen mechanism (just send the server fake hit box information off in the middle of nowhere).