Different currencies were simply different weights of physical gold, and the exchange rate between one nation’s currency and the other was the simple conversion between different weight units, as straightforward as converting inches to centimeters.
In the same way metric and imperial units are just a way to measure the underlying length, national currencies were just a way to measure economic value as represented in the universal store of value, gold.
Each country’s money supply was determined by the natural working of the market system.