Robinhood sells your orders to Citadel. This controversial practice is known as "payment for order flow."
Like true scumbags, Citadel actually urged the SEC to ban this in 2004.
"Citadel Group urges the Commission to ban payment for order
"This practice distorts order routing decisions, is anti-competitive, and creates an obvious and substantial conflict of interest between broker-dealers and their customers."
"Broker-dealers accepting payment for order flow have a strong incentive to route orders based on the amount of order flow payments, which benefit these broker-dealers, rather than on the basis of execution quality, which benefits their customers."
"Furthermore, the parties making such payments (either voluntarily or through an exchange-mandated program) are forced to find other ways to recoup the amounts of such payments..."
"...whether through wider spreads or a reduction in other benefits that otherwise could, and should, be provided to customers."