The United States has always been a country of immigrants. During the late 1800s through the market crash of 1929, ten percent of the country moved to the country every decade. After the Great Depression, immigration slowed to a trickle. It has grown to about 3.5% of the population per decade today.
Consider that the US grants more citizenship each decade than the entire populations of most countries.
The main changes to the immigrant population have been the source countries (now Asia and Latin America rather than Europe and Russia) and the greater number of illegal immigrants that are also coming to the US. Over 80% of today’s illegal immigrants come from Mexico and Latin America.