Exchange rates are indeed set by markets, but then all a central bank has to do to devalue its currency is to sell its own currency for another (or buy assets denominated in another currency, which is essentially the same thing with extra steps)
At the crudest level, what do you suppose would happen if the U.S. Treasury/Federal Reserve suddenly decide that every dollar-denominated account of $n got a "stimulus payment" of $2n?
That’s a very indirect way to devalue a currency for foreign exchange purposes. Sure, you’d be massively increasing that supply, but it’s doesn’t necessarily follow that said supply would flow directly into forex markets.
Against what, and by what means?
(Perhaps naively) I'd always assumed the markets decide the exchange rates of (most) Western currencies against each other.
Obviously with a dollop of central bank intervention here and there(!)