There are places that have now received (in the span of a few decades) millions of immigrants, despite the countries themselves originally being in the order or 5-15 millions to begin with.

That's the equivalent of 32 million Chinese (or whatever else) coming into the US in 2 decades.

And some of these countries regularly receive hundreds of thousands of people a year still.

It might be nonsense for rural Idaho to conceive that, but it's something that goes on elsewhere.