How does population growth in other countries affect the US?
People tend to move more developed regions if they are under stress
Population growth is a natural phenomena. Countries that have higher population growth rates cannot add new jobs to their economy or skills to their people.
Sometimes education (especially higher education) becomes problematic in some parts in the world. Therefore, especially well educated people would like to move to other regions to have better income, to have better educational conditions to their children, to have better health care facilities, etc.
Sometimes natural resources limit well being of people/countries who have higher population growth rate. In this case, a war or a famine might force people to search for a better place to survive. Whether legally or illegally (sooner or later), such people move to developing countries (such as the USA).