So the Fed must get the interest from the bonds. What does it do with the money? Return it to Treasury?
buy more bonds.
Actually they don't, that's not how the bond market works.
They spent say, $2T for a stack of bonds with a total value at maturity of say, $2.2T. The 200B profit is what they might eventually get instead of interest. Or not. If the plan is to raise interest rates when sell-time comes around, they may want to sell the bonds at a loss as a way to kick rates higher.
fwiw, right now the fed's holdings is ten percent total federal debt. That's the same percent when Reagan was president. So much for QE money printing.