What is quantity theory of moneyIs it the theory about price level of goods and services is directly proportional to the amount of money in circulation, or money supply? Or do we need to talk about inflation rate and money growth rate?

Also Monetary Policy Transmission - buying and selling securities Can someone please explain the purpose of banks buying and selling securities in an open market operation and how that relates to monetary policy transmission? I'm consumed about how that then influences interest rates and more. Thanks