the funding could also come from Fed’s facilities, like the SRF or OMO however, current SRF rate is 4.5%, which is above the yield on T-bills, and Fed is still officially in QT, so no large-scale, longer-term liquidity injections via open market operations
moreover, currently the US Treasury is issuing debt and cash at ON RRP is running low. MMF, dealers and banks purchase those T-bills. if they do not have cash in ON RRP, it will be financed by outflows from bank reserve accounts into TGA