It CAN be VERY simple...
A bank receives deposits for people who want to earn interest on their money. For those that want to borrow money, the bank loans money and charges interest on the loans. The money earned from interest is used to pay interest to depositors, and also to pay for costs like the employees who work at the bank, the bank building, etc.
This is EXACTLY how banks worked a few decades ago, and exactly how most credit unions operate today.