$2.5 Trillion is held by the Fed & the remaining $800 billion divided among others including tax havens. The important thing to note is that China is gradually quitting US Treasuries (not increasing them) to finance its BRI & African/South American iron ore & coal development to replace Aussie supplies.
It is interesting to note that the price of iron ore has lifted while steel's other input, met coal, has dropped. This seems to be due to Brazil's drop in iron ore mining rather than overall increased demand from China across the board. Re-Bar price is a good indicator of China's economic stimuli because its use/demand is usually the first to rise in increased infrastructure activities in both China domestically & externally in BRI projects.
