Let me explain this point to you from a financial market perspective and capital flow perspective. Capital mobility means capital can flow freely in and out of a country and a flexible exchange rate policy is where the central bank does not intervene or regular the exchange rates. They are entirely market driven. Most developed economies of the US, Europe, and Japan have free capital mobility and flexible exchange rates.
You would have seen that monetary policy of tweaking money supply, borrowings and interest rates has been used by most of these developed economies in the last 10 years. In the short term monetary policy works and is easier to implement. But over the long term, the fiscal policy does work better than monetary policy. For more details on this subject, you can visit the following link: https://www.economics.utoronto.ca/jfloyd/modules/islm.html