Where does the Fed buy MBS from?

Could you please clarify for me where exactly does the Federal Reserve acquire Mortgage-Backed Securities (MBS) from? Are they purchased directly from banks or other financial institutions that have originated and securitized these mortgages? Or does the Fed engage in some form of secondary market transactions to acquire these securities? I'm interested in understanding the specific channels through which the Fed acquires MBS and how this process contributes to its monetary policy objectives.