The STP (Straight Through Processing) technology requires no dealing desk and is the model which is used by our Company. All orders are routed to the broker's liquidity providers, and prices are executed at the bid/ask rate provided by liquidity providers.
Liquidity providers in this case are hedge funds, big banks, and investors that effectively act as counterparties to each trade. Usually, the STP broker has an internal liquidity pool that is represented by different liquidity providers, that compete for the best bid/ask spreads for STP broker orders.
You may read more information in this article.