US is the "buyer" who has the money. China is the biggest supplier of goods. A statement like the above is overly simplistic. The US will exert power and influence in a buyer's market, and will lose influence in a seller's market. Which market we're in is an be interesting discussion, and it predicates any prediction of the future.

A country's demand is linked to its supply. After all, it has to pay for the goods it demands with something.

That's a very loose link in a world where fiat currency and reserve currency exist for printing money.

But not negligible. The USD wouldn't be where it is without the US economy.