Can someone help me understand HOW a bank is able to pay me for simply having money in their bank?

I have been wanting to open an online savings account. I have been learning about online banking, as the concept of not having a physical bank to walk in to baffles me a bit. I initially felt like the money I deposited would somehow be lost of 'unavailable' when I need it. Now I know that if said bank is FDIC insured then it's safe.

*[b]I understand[/b] that the higher the APY, the more I will earn in interest. I also understand that the lack of a physical location equals lower/no fees.

My question is: HOW is the bank able to pay me for banking there? This is throwing me for a loop.

The grocery store doesn't pay me nor give me any reward for shopping there ...🤷‍♀️