Is this really specific to Robinhood? If I go to https://investor.vanguard.com/investing/online-trading/order... and follow the "View the quarterly reports" link, it shows that Vanguard routes orders to these companies: Citadel Securities, VIRTU Americas LLC, G1 Execution Services, UBS Securities LLC, Susquehanna Capital Group, Citigroup Global Markets

Wikipedia says Virtu Financial is an HFT firm: https://en.wikipedia.org/wiki/Virtu_Financial

Edit: yet, the same report (as well as a document that the article links to) says "Vanguard Brokerage does not receive compensation for directing order flow in equity securities". So Vanguard routes to similar companies, but doesn't get compensation from them?