Why is America an immigrant country?

It is said that the earliest indigenous people in the North American continent: Indians migrated from Asia (more precisely, China) through the Bering Strait (at that time, Asia and the American continent were still connected, and the Bering Strait did not exist). Of course, this figure is relatively small and is not known to the so-called "developed" and "civilized" regions in Asia and Europe. Until Europeans (Britain first) exiled these criminals to the United States. Only slowly and resolutely began the process of emigrating to North America. Later, more and more European immigrants began to kill Indians (aborigines) and began to sell black slaves and Asian laborers. This is the second wave of immigrants in North America. Later, in the modern "civilization" stage, there was basically no war on the American continent in World War II (except Pearl Harbor), and a large number of refugees (especially Jews) from war-torn areas such as Europe and Asia kept pouring into the United States, which can be said to be the third immigration tide in the United States. In short, the United States is an immigrant country by descent, and it is relatively "atmospheric" in accepting immigrants (relative to European countries), because Americans can be distinguished not only by whites, blacks and yellow people, but also by Asian, African-American, Jewish, and even German, French, Spanish, English and other very detailed sources. Almost every American is a foreign immigrant from the ancestors, so isn't it easy to understand the country's open policy of accepting immigrants (at least for more than 200 years now, only a few generations)?