That's a wrong statement though.

One million people is a single city. You do have a problem if one million people are suddenly relocating. It has happened before for war, natural disasters or other reasons.

Migrating a billion people is not a thing, for the simple fact that there isn't any human settlement any close to approaching a billion people. If you're talking about the scale of a whole continent, migratory flux span over a lifetime and are not homogeneous.

I’m talking from the perspective of the country the refugees are trying to enter. If a million refugees are headed your way and you don’t want to let them in, you can close the border and enact strict immigration controls and you’re fine. Sucks for the refugees, but the target country has no problem aside from the emotional burden. If a billion people are headed your way, you’re fucked.

>Migrating a billion people is not a thing

Yet.