For people who work in banks, where does the money come from that's used for paying them?

They just take money as they need out of your account.

Most from interest paid by borrowers.

Fees, investment, interest on loans etc

> "I haven't heard about banks' customers paying their banks."

Yes you have. You're thinking only of checking accounts, but banks do a lot more than that. They make loans: mortgages, personal loans, even loans to other banks. The customers who borrow money pay interest (at a much higher rate than the bank pays you for your checking account).

Mortgage fees are higher than interest on savings accounts.

From fractional reserve lending.

The banks loan the deposits out and. Charge interest.