No one and everyone pays for it. Those sorts of quantitative easing moves are purely money creation. So not a penny of that ends up on the taxpayers, but then again, everyone with a dollar theoretically pays for it with inflation. But on the flip side, the velocity of money collapsed, so maybe they won't be very inflationary.
The transaction is that the Fed literally created new money and used it to buy existing bonds to try to juice the money supply. When the bonds mature, the borrower has to cut a check back to the Fed and then the Fed retires the money. It's just a round trip.
The worst thing that has happened to America in the past 20-ish years is the monstrous politicization of the Fed. They held rates insanely low for Obama. Trump browbeat them and they cut rates for him during a period of pretty sizable economic expansion, which is completely unheard of. And then JPow basically would do anything Trump asked when The Germ hit. The Fed has become a total fucking joke. But as the US debt gets bigger and bigger, the Fed will have to keep rates insanely low because raising them will lead to even bigger federal deficits.