Money printer go brrr is a beautiful meme. It encapsulates years of study of money into something so easy to comprehend even the masses can get it. There's just one problem; it's wrong.
We know what money is. It's simple enough, it is a good that fulfills three functions in an economy: a store of value, a medium of exchange, and a unit of account. In Japan that is the Yen, in Europe it is (for now) the Euro, and in the US it is the US Dollar. But not all fiat currencies are the same.
In the dollar's case, when the dollar was tied to gold it was easy to answer the question, "a dollar of what?" Today, it's not that easy. A dollar bill and a non-physical dollar balance in your bank account are both dollars, but they act very differently. They are different types of dollars, and different types of dollars fulfill the three functions of money differently.
A dollar bill can buy something locally with anonymity, while a dollar balance in your bank can buy something anywhere on the planet (where it is legal to use that dollar), without anonymity. These are two types of dollars.
As we consider more exotic forms of currency it gets more difficult. What about deposits in a bank? What about if that money is lent out to another customer? Or, if the deposit that is lent out to another customer already operates as collateral for a loan from a second bank which is then deposited into a third bank? How much of all of that is part of the money supply?
Knowing the supply of money is not as easy as adding up all dollar denominated balances. The many different types of dollars are going to act differently. For example, if I "printed" thousands of $100 bills, put them in a wood box, and buried it in a location no one could find, would we say that was increasing the money supply? What if balance sheet entries showed dollars that could not be spent? That is actually what the Fed is doing. They are printing money that can never enter circulation. This "money" isn't money at all, it is referred to as bank reserves.
You can see how it gets very complicated. The bottom line is there are many different types of dollars, each with their own characteristics. When the Fed creates reserves at the Fed, this is technically dollars, but they aren't fungible with the rest of the economy because they cannot leave the Fed. They have one use and one use only, to make banks more solvent via accounting. They are treated as dollars on the balance sheets, but they can't actually be used as dollars in any other way. You can't buy a sandwich with a reserve held at the Fed.
Money printer go brrr isn't correct. The Fed cannot print fungible money, they are printing inert reserves.