There is ONE special actor (in the US) that can create money out of the thin air: the Federal Reserve.
It alone can "buy" securities by crediting banks with money created out of nothing. Banks simply see a transaction with a dollar amount on their accounts within the Federal Reserve, and that's it. The money just appears.
The invariant "money can't be created or destroyed" holds for everybody else, including individual banks.
> The answer is national debt.
That's completely incorrect. The Federal Reserve can arbitrarily create (or destroy) money even if the national debt goes away entirely.