You raise two good questions — I, too would like to know:
** Where did China and Japan spend or reinvest the proceeds?
** Who bought the half-trillion in US debt from China and Japan?
If “we” bought back our own debt, it should show up on the Fed’s balance sheet, I would guess.
Since June 2022, the Fed has reduced the Treasuries on their balance sheet by $320 billion.
T-Bills (12 months and less) at +4% are a lot more attractive than they were at less than 0.5% last March.