They can buy any +/- any bank

I think it's more cost effective to start a new bank.

But why would they. It's a lot of trouble, and being a bank gives you the right to lend in money from the public.

It's still possible to provide payment services without being a bank.

I think they could more or less buy VISA and pay in cash, and that makes more sense if you want to make money on retail payments.

I am not advocating that Apple needs to buy a bank, just noting that they have resources to buy pretty much any bank.