That isnt the main reason. They do it to make money, or at least protect the money they have without overheating their own economy. The Chinese don’t buy treasuries to make friendly with the USA, they do it because they don’t have any better options to deal with their trade surpluses. There is just nowhere else liquid enough to park a few hundred billion as needed.

Why they fund startups is even more self interested, they really do want to make money, and believe it or not, it is much easier and profitable to do that through investing then to become a rogue state.