The Congress just spends. THey don’t care where the money comes from. They “deem” it into existence. The Treasury sells bonds to the Fed to get the funds to keep the government running, as according to whatever continuing resolution they are operating under.
At least some of them must know where it comes from: the American taxpayer - a shrinking constituency. What do you suppose the collateral is on $21 tril + debt? Your job, your house, your children, your retirement... Sooner or later it has to come down to this.