You've got it totally wrong. The U.S. Treasury does issue treasury bonds in order to "pay" for government spending such as the stimulus checks, but who buys these bonds? It is definitely NOT the social security trust fund since it has no actual money at all. A tiny amount could be purchased by foreign governments but that is especially small right now where every country in the world is suffering economically. The Federal Reserve (Fed) is the primary buyer of these bonds and it creates the money to buy them from literally nothing - it doesn't even print it, they are just electronic entries on their balance sheet. This brand new money is used to buy all sorts of assets, not just U.S. treasuries (but mostly bonds of various types). The Fed has recently been buying up corporate bonds, real estate bonds, and government bonds at a very, very fast pace. See the
Fed's balance sheet for a look at how much and how fast. Any increase in that chart is newly created money out of thin air. Sometimes the Fed worries about inflation and sells back the bonds they've previously bought to remove money from circulation - the money they get from selling their assets is removed from the balance sheet and ceases to exist. But like the stock market, the trend is always upward for the Fed's balance sheet.
Another important question is who benefits from having sold those bonds to the Fed? The seller of course! Besides the U.S. Government, most of the other kinds of bonds they buy are owned by commercial and investment banks who are the very same people that make up the Fed! Yes, this is the fox guarding the hen house. They simply figure out which of their assets are trash (hint: corporate bonds of companies that will be going bankrupt soon and commercial real-estate soon to have no tenants) and these are the bonds that the Fed buys (and does so quickly enough to raise the prices well above real market prices). If for some reason the bank has loans against real estate that haven't been bundled into bonds, then the banks start bundling them up in order to dump them on the Fed. This is happening right now. These banks are owned and operated by very wealthy people who well know how to enrich themselves and their friends. Nearly all of this money in one way or another will flow into financial assets owned by them in order to prop them up. That is why the stock market is soaring right now despite the economy itself tanking.
