The word you’re looking for is ethnogenisis: https://en.wikipedia.org/wiki/Ethnogenesis. There is no “American” ethnicity, though “white people” might come close to being considered a synthetic ethnicity resulting from the immigration restriction and birth rate boom from 1921-1965.

But even that is too broad. It might be more accurate to get even more granular. For example, you might identify someone like Tim Walz as belonging to a synthetic Scandinavian-Midwestern ethnicity: although he has no actual Scandinavian ancestry, he grew up in Minnesota in what’s a recognizably distinct ethnocultural subgroup.

A far more useful analogy might be that “American” is a college football team.

The US did undergo ethnogenesis; particularly in the southeast, there have long been large numbers of people that identify their ethnicity as "American." The process was largely disrupted/reversed in the northeast with the Ellis Islander waves and then near-totally nationwide in recent decades. (The west was too new and too churny to have undergone anything like that.)

"African Americans" certainly also separately underwent ethnogenesis, although the preferred nomenclature there has changed, and there wasn't really any disruption there. But I think it's certainly fair to count them as a distinctly and uniquely US ethnicity.