Treasury raises money by issuing Treasury Notes/Bonds at regular intervals. These are bought by investment banks and superannuation funds etc ("the public"). International outfits are also quite keen on buying T-Notes/T-Bonds.
The US government/Congress uses this money to pay for things when tax receipts are insufficient. I think that there was a brief period during the Clinton administration when there was a government budget surplus, but not since.
This is called fiscal policy. and currently Janet Yellen is the boss at Treasury.
So far, all this has been done quite independently from the Fed, although the Fed does buy T-Notes from the public for a variety of reasons.
This is called quantitative easing, as a part of monetary policy where Jerome Powell is boss. This is independent of government/Yellen control.
The Fed and the Treasury are not allowed to transact directly with each other in the buying and selling of T-Bonds and T-Notes with each other, although the Fed, as the central bank, may have some operational duties.