You are thinking of a nation - and even a single bank - as a closed system. But this is an oversimplification that makes it impossible for you to make steps in your understanding of how this all worked. I've worked for a bank, both on the 'banking' side and on the IT side. One of the first things that gets drilled into your head is that banks create money. With every loan on the books more money gets put into circulation. There are some restrictions on how much you can put into circulation and there are some restrictions on how much cash you have to have on hand compared to the number of deposits that you have lying around.

But a bank could easily (as long as the bank is 'solvent' according to the local central bank) write a loan will in excess of it's deposits, technically it need not have any deposits at all.