I think this thread needs an ELI5 for how the whole process works from the ground up as there seems to be a lot of popular misconceptions.

This Planet money episode addresses the whole topic of where the Fed is getting all the money from, and how inflation fits into the picture. I found it pretty helpful.

https://www.npr.org/transcripts/821787090

That would be incredibly useful

That's a best case scenario and feels overly optimistic. What are the chances it gets fully repaid?

Assuming society continues onward, it doesn't really need to be fully repaid. The value is in the organization and structure of society. However, war, environmental catastrophes, population decline, and loss of trust between citizens of a nation can cause a decrease in the predictability of actions and consequences in society, and that would be a problem for any debt issued with rosier assumptions.

It cannot be repaid, ever, because the interest exceeds the amount of money that is currently in existence, which means they will always have to borrow more money (which is created out of thin air) to service the debt.

Interest on the national debt is $400-$600 billion per year depending on what you count (a lot of it is paid to the federal government itself). It's a lot of money, but it's a small fraction of the federal budget. It's also less than the amount of money in existence - M1 is $5.5 trillion and can easily be expanded, as mentioned.

(It also wouldn't necessarily matter if the interest per year were greater than the total amount of money in existence. If the money supply were $20, that wouldn't make it impossible for me to pay you $21/year; I could pay it in 21 $1 payments.)