Until Robinhood came along I thought transaction fee was how brokerages make money. It really was a surprise to me when these guys made it zero. I still don't understand how they make money though.

On a different note, I don't think they are in anyway different from the other brokerages, the only reason these guys became popular is because of the zero transaction fee trick. It was a good one but there might not be more tricks left for new entrants. The software is written from scratch in 2018/19 so obviously it will take the shape of that time, nothing great about that.