Since this question always comes up, I'll aggregate:

- Payment for order flow - Interest on idle cash - Robinhood Gold, a subscription model with side benefits, the most notable of which is... - Margin lending

It's unclear whether Robinhood is profitable or has a road towards profitability since none of these are really unique in the retail brokerage space, but it certainly seems possible that these combined with a huge customer base and the right amount of frugality could allow for it.