Citadel, who is on the losing side of the short GME trade, is who Robinhood sells order flow to. Make of that what you will.
https://finance.yahoo.com/news/citadel-point72-back-melvin-2... (Citadel, Point72 Back Melvin With $2.75 Billion After Losses)
https://www.cnbc.com/2020/08/13/how-robinhood-makes-money-on...