China has few natural resources. Especially when considering their need. They have a huge population. And they export vast amounts of goods. So, they need to import. Imports require dollars. Oil alone is a big reason for dollars. They buy oil with dollars. Remember China is the largest new car market. Most people in China did not own a car ten years ago. Now they drive and consume gas. Besides oil, most commodities China needs are traded in dollars.
So dollars come in and go out. Right now they are supposed to have $2 trillion dollars. But they have money flowing out in personal bank accounts. They buy resources and energy from other countries. So the $2 trillion is getting smaller. Its not growing. Japan is now the largest holder of US debt. Thats because China has been selling our bonds for dollars.
So much is going out now (capital flight) that China is attempting to stop it.
Good luck with that.
When the new wealthy of China see the purchasing power of their devalued Yuans cratering they will naturally try to do something about it. That would include exchanging it for physical currency such as gold or exchanging it for other currencies or traded securities.
Japan once had that problem, too. They tried to solve it by trying to take over all of Asia.