Where does China get the money to buy US debt?

There is so much talk about China buying American debt, there’s little info on how the Chinese can actually afford to buy up this debt. Where is there money coming from? Aside from cheap labor, what are they selling that’s giving them this leverage. They are not leaders in technology, medicine, innovation, stuff that generates money.

China earns very little money from exploited labour by capitalists from america, japan, korea, taiwan, and hong kong. they are not leaders in technology, but do have significant satelite, steel, industry. they also make other technological stuff such as broadband, internet communications. of course, you would not know this because you live in an oppressed capitalist society where your media is censored.