In the traditional financial (TradFi) trading market, brokers are able to offer zero-commission trades to retail clients because high-frequency trading firms such as Citadel Securities, Susquehanna International, Wolverine Trading, etc., compete to execute these order flows. This is known as "Order Flow Payment (PFOF)".

These companies are willing to take on these order flows in large quantities at close to mid-price prices because they are, by definition, lacking inside information. There's a lot of literature on why order flow payments are good for the world, not bad (although it often has negative connotations).

The problem with order flow payment models like Robinhood and E-Trade is the lack of transparency and the fact that auctions are limited to market makers that the broker works with. In addition, there are layers of intermediaries such as clearing houses, exchanges, brokers, etc., all of which draw fees, which are hidden from the end user and are often included in the bid-ask spread.