How do personal finance banking apps make money?

There are a bunch of apps and start ups, like Qapital and Varo that helps you save money. Some of them has monthly fees, but not all of them. How else could a company like that make any profit, if their only service is saving accounts and using their debit card?

Is there a way they could earn money by using their debit card? Or would they invest the money on the saving accounts into safe assets, so they could make more profit?