So the gist of national debt in my understanding is Nation A asks for aid of some kind from (or commits unintentional damages to) Nation B who later on deems Nation A owes them based on their interpretation of the ordeal, with varying layers of complexity.

Cities do that to each other too, right? Why do we never hear of a town or city urging that another town or city owes them, whether they’re both in the same country or not?

And/or better yet, why doesn’t national debt default on that kind of small level, where, instead of India saying “you owe us money, Israel”, if all the money is coming from New Delhi and going to, say, Jerusalem, the mayor of New Delhi can just say to the mayor of Jerusalem “hey, get your billionaires to pay up”?

I know cities are not as sovereign as countries, but they’re still valid units, correct?