They aren't really printing money. They are buying up debt, both private and government that will eventually be sold back to the market or will mature this returning that money back, with appreciation, to the fed. Which will then make it disappear just as easily as they created it.

Sure. The Fed doesn't even own printing presses, the Department of Treasury does. However, in this context, "printing money" doesn't mean making physical pieces of paper. It means creating money out of thin air. It does that by buying debt, which, as you note, can be destroyed just as fast as it is created.