Not necessarily, this can happen with immigration alone. For example, French and English descendants used to form a much greater percentage of the Canadian population, yet they were not "ethnically cleansed".

Yes, a flood of immigration combined with systematically displacing the local population. See the Nakba and Settler movement.

Doesn’t it depend on the time scale?

If we are talking about pre-globalization many countries are no longer have a majority “native” population. (US, Japan, Taiwan, parts of Europe) and that is just the reality of human history.

I think we have to decide on a time when the back and fourth geocoding between groups is no longer acceptable. Most of the world thinks it’s around the end of ww2 and the start of globalization, but are you contending that it should be later, and the Arabs should take back Israel?

Arabs are not native to that area.