An immigrant is someone who comes from another country to an established country, and intends to live there. If they come legally - according to the laws of the target country, they are legal immigrants. If they flout the laws, then they are illegal.
We are not a "nation of immigrants." Most of us descended from immigrants, but WE are not immigrants. The descendants of enslaved people did not descend from immigrants. Different category altogether.
Indigenous people are a whole 'nother discussion.
Many of us are descended from SETTLERS - people who came here before the United States existed. Again, that is a whole different category.
Not that it matters. What matters for the purpose of discussion is that there are roughly thirty million mo-fo's in this country illegally, and the political Left intends, in due course, to make them all naturalized, voting, Democrat citizens. They contend that "we can't just send them all back home." Really? It was done before. Look up "Operation Wetback."
And not enough people care.