Through immigration, which almost every developed country is trying as well?

Yes!

But who is better at it than the USA ?

Things change very fast when things cannot be fixed in any other way. I think you underestimate how quickly things reverse one way or another. Look at Japan's foreign residents number YoY, a nation that have actively hated (hates?) foreigners, but understands how we're going to be depended on them for the next decade due to low birth rates.

When every nation fights against each other to prop up their population numbers, immigrants will choose the place that's the best for them. Think of it as product competition, and consumers are the immigrants that pick what's best for them.

> immigrants will choose the place that's the best for them.

Immigrants have always done this, that’s why they emigrate in the first place