The treasury sells bonds to the world and the Fed manages the proceeds.
“The world” not buying enough treasury bonds to keep up with recent deficit spending...the Fed now buying treasury bonds themselves with freshly created $$. (”Monetizing the Debt”). Being done thru Fed member banks who buy the treasury bonds and “sell” them to the Fed in return for a “chit/IOU”...that the banks can put on the books as “reserves”.
The only reason we get away with this is that the US Dollar is the world reserve currency. At some point that will come to an end and then no one will be willing to buy our debt. The government will devalue the dollar. The result will not be pretty. See: The Weimar Republic.