Commercial banks don’t hold “securities accounts” at the Fed. The Federal Reserve Bank of New York has a “System Open Market Account” in which it holds securities that the Fed has purchased in Open Market Operations.
The Federal Reserve’s balance sheet shows that its liabilities do not include “securities accounts” held by commercial banks. Commercial banks hold “reserve balances” with the Fed. For the week ending on Sept. 11, these were $2,265,831 million.
The balance sheet shows that the Fed’s assets include “Securities Held Outright.” For the week ending on Sept. 11, these were $3,394,176 million.
Here is a description of how the Federal Reserve Bank of New York conducts open market operations.