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

Because immigrants are brown and poor.

It’s not a British/American thing - any nationality can be referred to as an expat. It’s all a matter of what you’re trying to emphasize. The term “expat” implies being in a different country and feeling like a foreigner - using the term suggests that there is a degree of culture shock or not feeling like you fully fit in. Foreigners will often look for expat communities for support. That may be why you’re noticing it with British and American foreigners - you can be a French expat or a German expat or any other nationality, but if English isn’t your first language you’re less likely to know the term.

You’re also less likely to hear an American or British person refer to people who come to the U.S. or U.K. as “expats” - the term "expat"implies inclusivity with other people who came from the same place, while “immigrant” carries the implication of someone from a different culture that came here. As a native English speaker, I would think it sounds perfectly natural to hear someone say “I’m a Syrian expat”, but I would only use the term to describe “the Syrian expat community” (i.e. the Syrians that have come here and are relying on each other for support). If I were describing the same person, I would say “Syrian immigrant” because I’m not the one feeling the culture shock of being in a foreign land. (or I would use the term “refugee” which carries the implication that they’re here, but not by choice - they were forced out of their home)

“Immigrant” often also implies some sort of formal legal status, although in a looser sense it just means that you live in that country on a permanent basis. All immigrants are also expats, but not all expats are immigrants.

As others have pointed out here, while neither term is by itself positive or negative, “expat” will almost never be used in a negative sense, but “immigrant” can be used in a derogatory way, although it can also be neutral or positive depending on the speaker and context.

I’ve always interpreted it as someone who’s here only temporarily. They plan to go back eventually and as such stick to expat social groups with similar issues for e. g. temp permits as opposed to permanent permits for immigrants.

Because “immigrant bad” and they don’t see themselves as bad.