They wouldn’t default. They’d simply print money to cover the difference, deflating the value of your currency. Obama did this quite often. Biden will likely do this leading into the election to bring down interest rates (rather than cutting spending, which would have a similar impact withou lessening the value of a dollar). Buying up the bonds that are not purchased by others in an attempt to keep interest rates artificially low.
if you owe the bank 20 grand, you're at risk. if you owe the bank 200 million, the bank is at risk. the chinese are in mediocre financial shape and would bring the building down on themselves if they pushed us towards default.