China, for the last several years, has typically been the largest holder of US debt.
They're the largest foreign holder of US debt. But about 2/3rds of US debt is owned by domestic entities--and of domestic holders, Social Security owns the most, at about 16%. How does the US government own its own debt? Who knows? But in any case, China is a couple steps down the list; it owns about 7% of US debt.