A traditional bank used to take people's deposits and loan them to other for a fee. the fee would then be returned on aggregate to depositors less the cost of business and profit. this is so called fractional reserve banking.

But that doesn't really provide that much interest to depositors, in this age of loose money supply.

So there are more exotic functions that "investment" banks fiddle with. Ie trading on the stock market (regulated betting) buying companies, and trading on futures and other pure bets.

The problem is that banks are bigger, and have more depositors. So when one pops, other go because they are all doing the same shady shit to make profit.

In the UK there are still a few building societies that offer traditional, boring, consumer banking and mortgages. But they are now large national behemoths.

But to your point, Brex is a payment manager/facilitator. Its not really offering banking, its more a service to manage expenditure. This might seem like pedantry, but its different enough to make the point.