This Kahn guy is great. Here is his lecture on how countries maintain trade imbalances by printing currency and using that printed money to buy the excess dollars that a trade imbalances creates so that their prices remain lower than ours thus maintaining the imbalance. Then they take those extra dollars they bought with printed money and buy our bonds so we pay them interest. And the sad part is we allow them to do it. Probably because we elect people who don't understand what they are doing. Or worse, they don't care. Take your pick. Neither one is very flattering.
https://www.khanacademy.org/economics-finance-domain/core-finance/money-and-banking/china-us-debt-situation/v/floating-exchange-resolving-trade-imbalance