There are to some extent two questions here, one being how was activity within then country paid for, and the other how were imported weaponry and services paid for.
In totalitarian societies the first becomes somewhat immaterial, since it only means printing money that people are forced to accept. It's somewhat more complicated in democratic societies, though a wartime command economy has much in common with totalitartian regimes, including, e.g. forcing price controls and rationing.
How foreign countries are paid off is different, because you need either assets abroad to sell or as collateral for loans, which Britain had plenty of at least in the beginning, which was one of its big advantages as against Germany, which despite Schacht's best efforts had very few assets abroad, and relied on getting other countries to accept German paper.
It's a biggish subject however, and offhand I can't think of a good source.