I think both Robinhood and M1 Finance collect interest on their customers' cash and securities. Robinhood also makes money by offering margin to its customers a

I think both Robinhood and M1 Finance collect interest on their customers' cash and securities. Robinhood also makes money by offering margin to its customers and I believe M1 Finance has a similar but more general offering called M1 Borrow where the money borrowed can be used anywhere instead of just trading.