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Glibly, all of it. Immigrants make up 98.9% of the United States population as of the latest census.

If you continue to extend your time scale, immigrants make up 100% of the US population.

(Assuming that "immigrant" != "first-generation immigrant, of course, which seems to be in line with your comment.)

That's true of all non-african countries.

It's probably true of most African countries, with the possible exception of Kenya and the surrounding countries.