Are all American countries immigrant countries like the United States?

Almost, but the proportion of aborigines varies from country to country.

Before Europeans arrived in America, there was already human life on the American continent, and Central and South America created splendid ancient civilizations. When Europeans arrived in America, they called these native Americans "Indians" because they didn't know they had arrived in a new continent and thought they had arrived in India.

After Europeans came to America, they began barbaric plunder and slaughter, and then began colonial rule. In the cruel plunder and slaughter, the population of Native Americans has been greatly reduced and their culture has been destroyed.

Because of the terrain and natural environment, the aborigines in North America almost disappeared, so immigrants from the United States and Canada and their descendants account for the vast majority, mostly immigrants from Britain and France, and a few immigrants from Spain and Portugal and later Africans, which are typical immigrant countries. South America, on the other hand, is dominated by Spanish and Portuguese immigrants, retaining a considerable number of indigenous people, but also generally accepting western civilization. Today, almost half of Central American nationals are immigrants, half are indigenous, and most of them are mixed-race. In South America, European immigrants account for a large proportion, but they are mainly mixed-race. There are not many pure European immigrants and their descendants. Basically, it can also be called a country of immigration.