Who decides foreign exchange values and how do they do it.

In: Economics

Its supply and demand. As people buy a currency its value rises, as the sell it the value decreases.

If you mean how do you determine the long term value (so expected exchange rates) the simplest reasonably accurate predictor is interest rate parity. This is :

Future rate = current rate x ((1 + interest in domestic currency)/(1 + interest on foreign currency))

but it is an estimation, not an accurate prediction