This is a complicated subject as the other posts show. I only know little about it.
A few things:
While countries borrow a lot, most of them do not lend as much. They borrow from individuals, corporations, and international entities such as the International Monetary Fund (IMF). Lately, stablecoin issuers such as Tether (USDT) and Circle (USDC) have been buying increasingly large shares of the debt to back the tokens they issue which stay nominally very close to $1 in value. These assets back the coins while allowing them to earn massive interest returns.
Central banks (the USA Federal Reserve) and others also own and buy large amounts of hard assets, the main one being gold. They are fully aware that their fiat currencies are the exact definition of Ponzi schemes!
I took one semester of macro economics in college. What I learned was that economists have a lot of fancy math and models that work great until the next day’s news comes out. LOL
Fiat money systems go back at least as far as the Roman Empire and every one of them without exception has collapsed - usually within around 50 years. The current generation of them have already exceeded that average and are destabilizing.
I could address a number of the other concerns raised in this thread, but it would be a lot of work and a very long post.
If you want to understand it all more, the best place to start are the books by Saifedean Ammous. The first one, The Bitcoin Standard, despite the name, spends the first half of the book explaining what money is and how it works. It is easy to read and very well written.
Neville, the reason economics didn’t make any sense to you is because it’s current form (Keynesian and its derivatives) was invented specifically to hide the fact that nations had spent huge sums that they didn’t have to finance WW I and WW II. It was never a designed as a real theory.
The real stuff generally goes by the name Austrian economics and once you study it even casually, you won’t believe how insane and evil our current systems are revealed to be. They were designed specifically and intentionally to rob the poor and enrich the rich.
Just in case someone wonders, this has nothing directly to do with capitalism vs. communism… Austrian economics requires capitalism and free markets to work.