The 14th Amendment, ratified in 1868, created birthright citizenship. Anyone born in the U.S. (with some exceptions, like children of foreign diplomats accredited in the U.S.) automatically becomes a U.S. citizen. It also covers people who become citizens through naturalization.
Section "Learn and Explore" uses official instructions and media files of US Citizenship and Immigration Services, US National Museum of American History, The Smithsonian Institution and other governmental and public organization.