In theory, An expat is someone sent abroad on short/mid-term mission while working for an employer, while a migrant is someone moving abroad to find a job and sometimes to start a new life.

This means that, if let’s say if a Mining company in Ghana sends someone to New-York to be the “US sales director” that person would be an an expat While an American opening an hotel in Ghana would be an immigrant.

But indeed, in many cases : Expat = European/North-American, Immigrant = From somewhere else