[RobertoG]

"2) Bob takes out $900 loan from Bank X and deposits it in Bank Y"

If Bob takes cash from Bank X and deposits in Bank Y, Bank X has less cash, that movement is reflected in their balance and in their reserves.

If Bob makes and electronic transfer from Bank X to Bank Y, at the end of the day, Bank X and Y have to clear their balance with each other. As some people have moved money in the opposite direction, from Bank Y to Bank X, the balance could be compensated.

If for some reason, people only retire money from a bank without never making deposits, you have a bank run and it's an indicator of mistrust in that bank.