Repurchase agreements (the 1.5T) are a swap of one form of government debt (Treasury debt) for another (Federal Reserve balances). 1.5T for coronavirus response is new spending and must be approved by Congress.
The Fed is increasing the amount of money it lends to entities which buy and sell govt debt. This will keep the market for Treasury debt working well. It would be much simpler to just have the Fed print money when the Treasury would like to deficit spend.
However, Congress didn't like the optics of that when the Federal Reserve act was passed, so they go through some rigamarole to 'create the conditions' so that Treasury debt always has buyers in the market place.
The Fed lends to government bond dealers knowing that they will use the funds to bid at Treasury auctions. This creates the 'illusion' that the Fed is not funding the Treasury. Bond dealers then sell the bonds to investors, hold some in inventory. Charge fees, ect.
Final point. Under the system we have, nothing stops Congress from appropriating 1.5T or whatever amount it likes to fight the coronavirus financing it entirely with debt. The Fed will again make the accommodations so that the bonds sell in the market place at reasonable prices.
But, Congress, not the Fed, has to act.
