Humanities & Cultures

I was pondering differences between endonyms and exonyms, but then I started wondering where demonyms really, originally come from. I imagine it might come from the name of the leader of a group of people, that name becoming associated with that group as a whole and when people start mingling more it becomes a name associated with a whole people and so it dominos on, but if anybody has any good reads regarding it I'd be interested to check it out. I know that demonyms often come from place names but that also begs the question how those places got their names to begin with, or if its more often than not just a matter of "you call that place that hence we will call it that" à la Sahara Desert for example.