Are only people of the US citizens?

Depends on how you define 'people of the US'. People living in the US are not by definition US citizens. They can be nationals of other countries if they stay on a residence and/or work permit, or they may be illegal aliens.

Others may be citizens of the US (for instance, by being born from US parents) but live outside the USA all of their lives and even also be the citizen of Another Country at the same time.