Physical currency in existence is a very negligible sum of all money. This is thanks to what the article hits on - endless interest, and endless debt. If you took every single dollar in existence, you wouldn't be able to make whole even a small fraction of all debts. Interestingly enough, even if you took every dollar, digital or physical, in existence that would still be true. Isn't interest interesting?

The primary way money is made is through loans, though the Fed buying treasuries also creates large amounts of money. When you deposit at a bank, they're only required to create a fraction of that deposit available. Hence - fractional reserve banking. [1] You deposit at a bank, they take that deposit and lend it out to somebody who now spends that money which eventually ends up getting deposited at a bank again, and the process iterates. If at some point everybody demands their money, a run on the bank, which the bank could not afford as they've lent out deposits to other people, then the Fed bails them out.

The exact process is slightly more subtle, but not fundamentally different. There's a lot of great videos on a search for 'how money is created' at any video site. Because it's a question few know the answer to, everybody wants to know the answer to, and the answer itself is somewhat unsettling.

[1] - https://en.wikipedia.org/wiki/Fractional-reserve_banking