Ask HN: How do bank ATM fees work?

So banks charge ATM fees for taking cash out in the U.S. if you aren't using that banks ATM card. Usually $2-5 is what I see anymore.

My bank reverses most of these fees for me every month although I rarely have any.

So a few questions to anyone who knows.

1. What does it cost the bank to do this transaction? I am assuming it is pennies.

2. Is the transaction basically an ACH or some custom method?

3. Is it really two transactions? One for the cash withdrawal and the other the fee? Otherwise how would my bank know how much to reverse?

4. I know there are different ATM networks so I assume some portion of the fee covers access to that network but out of $3 is it $.10 and the bank keeps the rest or ?

Thanks!