They sell order flow to exchanges, invest dormant money, and offer premium services like Robinhood Gold that give you faster settlement/deposit times as well as higher margin.

It isn't always possible to tell if you're getting a perfect fill on market orders due to the spreads changing so rapidly and Robinhood not offering live quotes. If I ever trade on Robinhood, I always have another account pulled up with a different broker where I can view live quotes. I also never use market orders