Debt between sovereign states (or separate currency unions) is a peculiar creature.
We can take for example a late 1970s year in which Japan was pouring econo-cars into the US market, US car companies were in deep trouble, and it seemed like every VCR, TV, and eggbeater had a Japanese name on it. Due to the habit of Japanese not to buy foreign goods if they could avoid it, we had a trade imbalance, and this was 'funded' by 'borrowing money' from Japan. What this meant in particular is that the Japanese central bank would purchase US Treasury Bills, flooding the US and eventually global economy with Yen.
If we look carefully at what that means, it means that the US printed paper IOUs furiously while the Japanese poured their efforts into making 'real things': cars, TVs, etc., and shipping these to us to consume.
Japan currently holds about $1 trillion in US debt (China, at this moment, has about $1.3 trillion). If Japan sells this debt to Saudi Arabia, it would be most likely in exchange for oil - something 'real'. If Japan were to demand 'repayment', the only way that could be done is for Japan to purchase real goods from the US - aircraft, wheat, beef, oranges, CPU chips, lumber, or whatever. At that point, dollar denominated debt would be repatriated in exchange for dollar denominated products.
Needless to say, anything bought from America is not produced in Japan. Presumably anything the Japanese could produce would never be bought from the US. Therefore, Japan would be creating jobs in the US if they demanded 'repayment'. Probability this will happen: zero.
In short, if the US sells debt to foreign countries that 'protect' their domestic industries from foreign competition, they will tend to accumulate foreign debt which they will never redeem, unless they get in the kind of trouble where they need massive imports.
Another peculiarity of all this is that the Japanese have been big savers. Therefore, the combined debt of the Japanese government and the American government are 'assets' to Japanese households. These 'assets' tend to be kept under the mattress, figuratively speaking. In short, money is earned, much of it is stashed away and never spent.
One interpretation of what money is is that it stimulates production and consumption. Therefore, as the US spends money like water the restaurants are full of waiters pocketing tips that are using it to buy gasoline which the station uses to buy gas which the oil company uses to run refineries, etc., etc. If the wage earner chooses in instead to park it in a safe, it doesn't stimulate commerce. If it sits in the safe until the 20-something retires at age 55, that currency does not act as money over a period of, say 30 years. That is 30 years of potential vendors not working and not earning those yen or dollars.
Therefore, as the US 'prints' money and puts it in circulation, foreign debtors remove it by 'saving' it in their respective central banks. This is also a problem within the US: wealthy people also 'hoard' cash, so to speak, rather than keeping it circulating in the economy to stimulate further activity.