Answer:
Japan and China are the largest foreign holders of US debt. By January 2018, China held $1.17 trillion of US debt, which is a lot of money. But it has decreased to $1.1 trillion by January 2020 (still is a lot of money).
Current American national debt is approximately $23.4 trillion, so China's share is almost 5%. Currently only Japan owns more national debt than China, with $1.17 trillion approximately.
What both countries have in common is that the US has a trade deficit with them. Logically we can assume that countries that have high trade surpluses with the US, will also have large amounts of US securities. Both China and Japan benefit from trading with the US and instead of taking money, they take home US securities. The financial outflow to those countries is larger than the financial inflow from those countries.