There are three sources of getting the stimulus money by the Treasury Department
- Public Sector
Like other parts of the government
This includes companies, businesses and other investors
This includes the sources from other countries which invest in the US
You forget the 4th source: the Federal Reserve.
It's likely the sources you mention cannot absorb the securities, therefore the Fed has to "print money" via Open Market Operations (OMO) and buy it.
https://www.investopedia.com/terms/o/openmarketoperations.aspIt's recommended to watch the video till the end, both for OP and the doomsayers here to know what the "printing money" is all about.