Earlier quoted context omitted.
Although robinhood's app was pretty revolutionary, I think their real "innovation" was removing the cost of trades, basically forced the entire market to remove that. Before, it would cost 4.99+ to execute a trade, so if you were not buying significant amounts that could EASILY eat up a lot of profits.
It’s looks like free but it isn’t, RH makes money by selling trading requests to high frequency trading firms like Citadel, which buy/sell before the actual order in a better position so they can earn a penny from selling to or buying from you. The more transactions, the more they earn. - https://fortune.com/2020/07/08/robinhood-makes-millions-sell...
That's literally not payment for order flow works, and it's illegal under regulation NMS.