Dealing desk brokers, market makers, same thing terms to describe the same guys effectively. As one of the names imply, they make a market.
Dealing desk brokers match client buy and sell orders, and they fill the remaining orders. This is how a market is made. How does the broker make their money if they are literally connecting a buyer to a seller?
They do not make their money by trading against you. They offer fixed spreads on prices because they can match buyers and sellers, they add a little bit on top (the spread) and that’s their money.
Think of the spread like a finder’s fee.