“Printing money” is an easy way to describe the result of the federal reserve purchasing debt. When QE occurs, the fed buys bonds and MBS. They credit the seller’s account, and the securities get added to the Feds balance sheet. They don’t maintain cash reserves for this its straight up a credit without a buyer side debit. QE literally adds money to the financial system…hence “money printing”

The covid QE response purchases securities at a rate of ~1.5 trillion per year.