But why is it successful? Where did their money come from? How sustainable is it? The core issues from capitalism still exist there, but they have more money with a smaller population.

I think it helps to draw some distinctions here. Nordic countries have strong social welfare systems, but private property and free enterprise are still a thing. This is not at all the same as communism where everything is ultimately state owned and operated.

China is an interesting example too because it's basically capitalist with strong government oversight. So you can go hog wild on exploiting labor and amassing wealth as long as you don't oppose the overall goals of the government. We'll see how long they can keep it running - the problem with most authoritarian systems is that they're only as good as their current leadership, and when that changes things tend to fall apart.