Does the bank rely solely on interest on loans from banks?
Yes you are right and that is the reason why cryptocurrency is a threat to the business of banks because it can actually decrease the people that are placing their money on their business which will slowly decreasing their own profits.
Yes. They lend your money to others, while sharing a very little part of the interest with you... The interest they charge from borrowers is much higher than the comission they give you back. Also, everything is secret: as a saving account holder in the bank, you don't have access to any detail about how your money is being used, how much the bank is really making with it. And I'm sure the reason behind this is to not create revolt among the bank's users.
Banks work with bureaucracy, shadiness, corporatism and protectionism, that is how they make money.