America would have a falling birth rate if not for immigration.
Its not just immigration but having immigration at the foundational core that helps blunt demographic cliffs for countries on this side of the world:
How a Vast Demographic Shift Will Reshape the World
Even in European countries where you see plenty of migration, you will still see a steep decline. Germany admitted that even with laxer immigration laws, they will still need to rely on automation heavily to blunt demographic decline impact.
It also helps that the US has destabilized dozens of countries and then takes advantage of the brain drain. I doubt that aspect of immigration policy would be mentioned in here though since its not rosy.