When you lose a particular deal, money goes to the broker and then the broker pays them to those ones who won the same deal which you have opened, or another one. It might be stated that there is a clear money rotation.
Broker earns money due to spreads and commissions, while all the money that rotate - go from trader to trader, every second and every closed position.
Something like that. Maybe there are other schemes of money rotation in this sphere, but I didn't hear something about other ones.