QT plays an important role. QT slowly changes supply/demand dynamics such that the TGA isn't soaking up more than it did with the Fed buying bonds.
The "QE replacement" is reverse repo, which softens the blow of QT, but with a disappearing act attached
RRP->TGA->QT
3/
It's not exactly this simple, but this is the basic explanation of the steps:
1. Treasury holds an auction 2. Dollars buy auctioned bonds with RRP cash 3. Dollars end up in TGA 4. Fed bonds mature 5. Treasury pays down Fed balance sheet