By assuming they evolve in the same way they've been evolving since WWII. While it may not look like it in San Francisco, but there are huge groups in the country, white and non-white that are having quite a few kids. Not enough to make population rise, but enough to keep it stable. It seems a vast number of people prefers a small city or medium city lifestyle with 3 kids. The weird thing is that this will make the experience of so many San Franciscans today a feature of the first half of the 21st century too. A hell of a lot of new immigrants will come from a pretty strict religious background and feel like releasing some steam in their early twenties. The influx of people like that should continue for another 20 years at least.

So the assumption is that the population of the US will continue to climb slowly due to natural growth, accelerated a little bit, but not much, by immigration.

Now if you look at the EU or China, they have a very different story. The EU has been stuck in a massive population drop rate (roughly approaching next generations being 50-70% of the previous generation for all of Western Europe, coupled with massive immigration numbers that make the numbers barely stable). That means that in a lot of countries in Western Europe, approaching 50% of the <20 population is of foreign origin, first or second generation, mostly from Northern Africa and the middle east. These numbers look scary, yet in the major cities, Paris, London, ... they look a lot scarier still. If the "unemployed youths that don't fit into society leads to revolution" crowd is correct, Europe has maybe 5-10 years left before that happens. There may be a further delay because of the simple fact that European mistreatment of the poor is still a lot better treatment than islamic nations' treatment of the middle class and the rich. So the immediate immigrants are actually quite content to be abused. Their children, however, are not.

China is in a pickle. They need, absolutely need, a lot of population growth to avoid disaster, but in absolute numbers everything they built is filled to the brim. They are trying to use the "one child policy" to spread out the population by relaxing it for ethnic Chinese in the outer regions, like Xinjang or Tibet. So if you are Chinese and want kids, if you move there and get a job there (Chinese control most businesses and employers are at least somewhat racist, so it's not impossibly hard), the government will let you do that, up to 5 kids in some places. The purpose is, of course, slow ethnic cleansing, but given that it's China, the UN will never even mention it.