Foreign individuals and businesses long ago cut back on their purchases of U.S. bonds. Their place was taken by foreign central banks. The central banks simply created money in their own currency and used it to buy our bonds.
The Federal Reserve always knew that we couldn’t rely on foreign central banks to buy our bonds forever. This may be the main reason it began the program called quantitative easing, in which the Fed created money out of thin air specifically to buy back U.S. debt.
Quantitative easing may have been intended to be a kind of insurance policy. If foreign central bank buying of U.S. bonds collapsed, the Fed would already have a program in place to buy them back itself.