When a country is ruled by monarchy who makes the decisions?

Their leader is the reigning monarch [ King or Queen ]. However, in a nominal monarchy, tradition allows for a monarch to remain as a figurehead, but the people are ruled by their own elected representatives.

For example, the Queen of England is the nominal ruler of Canada, but the effective power is held by the Canadian Parliamentary system [ a democratic institution which respects the tradition of royalty but without the monarchy having any real and effective power to rule the people of Canada ].