Exports received US dollars, and foreign investors invested US dollars in China, resulting in US dollars flowing into China. After it flows into China, it can only be used if it is converted into RMB. Customers sell dollars to commercial banks, and commercial banks pay customers RMB. After that, the commercial bank will sell the received US dollars to the People's Bank of China, and the People's Bank will exchange RMB for the commercial bank. The inflow of dollars eventually turned into RMB invested by the People's Bank of China. This is a simple "foreign exchange misappropriation" process. A large amount of foreign exchange means that the central bank has invested more RMB and the money supply has increased. The increase of money supply will directly lead to the rise of prices and develop into inflation. In order to prevent inflation, the central bank needs to recycle excess RMB. The central bank sells central bank bills in the domestic securities market, and institutions (such as banks) that buy central bank bills pay RMB to the central bank, which directly reduces the supply of RMB in the market. This is a simple description with many complicated details, but the basic principle is this.