> where does the cash come from if lots of people do the same?
the cash comes from the gov't printing it. It's a mere fraction of all spendable money. Banks store some amount of it, just for such cases where you wish to withdraw it.
If everyone demands their deposit as cash, the bank would run out of physical notes very quickly. They would, in all likelihood, ask the central bank to print the cash (in exchange for the reserves they hold at the CB), in order to fulfill the withdrawal. This might take days, weeks even, depending on how many notes are to be printed. Note that this isn't printing new money - it's merely transforming digitally stored money into physical paper.
> I wire money to the seller
Bank A would have an account within Bank B, and vice versa. At the end of the day, these banks "settle" their accounts; aka, if there's more money in Account A (in Bank B), it means Bank A has sent more money to Bank B, and Bank B needs to owe Bank A. This is basically how international transfers work. For local banks, it's likely that the Central Bank would clear these transfers up (aka, Central Banks are the banks for banks).
> what is stopping bank A from sending out an infinite amount of money to other banks?
The same reason why a bank cannot just create infinite money and spend it on hookers and blow. They are creating money only via lending, and there are laws regarding how much they're allowed to lend out (called reserve requirements).