system
A chat on subreddit /r/wallstreetbets
human
They take a negligible percent of cash. Since their model attracts more users the cut ends up being huge compared to some brokerages that charge commissions. 
  
 There also is no data to sell (that would be legal to sell). Everyone knows what Robinhood traders are doing because Robinhood displays hot activities in the app.

gpt
They absolutely sell order flow at the minimum