Explain why the United States is called the "land of immigrants"

In the United States, except for indigenous people and Indians of different races, people who are Americans now are descendants of immigrants from all over the world for hundreds of years, or they are immigrants themselves-for example, many people who became Americans from China in the past are immigrants. So the United States is the most ethnically complex country in the world. His people's organizations are related to people all over the world and all continents (Europe, Asia, Africa, Australia, etc.). ) and countries, as long as you can think of places and countries.