It's...complicated. Citadel (a fund) loaned a bunch of money to Melvin Capital which has a huge short position in GME. Citadel Securities, a separate entity, pays Robinhood a lot of money for data about its orders and helps execute these orders, while simultaneously using the data to make favorable trades for themselves. Citadel (the fund) and Citadel Securities are both owned by Citadel LLC, but supposedly operate completely independently with no collusion.

"Citadel owns Robinhood" however is completely false.