The majority of the citizens of the United States are not natives. Most of the US citizens are immigrants who came to the USA during the era of colonization. See how the United States was formed.
Many people migrate to America, or look forward to doing that, for various reasons including wanting to start a new life, wanting a better life or wanting to have more freedom in expressing their beliefs.
Americans come from different ethnic backgrounds. The fact that many Americans came from different countries resulted in making America a collection of different ethnic groups. See why do many people want to immigrate to the United States.
During times of food shortage and droughts, many people migrated to the United States to escape starvation. Immigrants came from different parts of Europe.
During times of droughts and starvation in China, a large number of Chinese citizens migrated to the United States in order to escape the food shortage. See also why the United States is called America.
A large number of Africans left Africa to go to America in order to live better lives and start new ones. Immigrants coming from Africa were given the name African Americans.
After the USA established, most of the immigrants were allowed in without many restrictions. The restrictions on immigration were added later.
Most American citizens are children and grand children of immigrants who came to the US earlier to seek a better life and better opportunities.
According to a 2013 report by the United Nations, 19.8% of the world's immigrants live in the USA. America thus has more immigrants than any other place in the world.