How do I correctly think about government domestic debt and how it influences real material things?

I'm sorry, I'm too dumb to even phrase the question correctly. I'm doing my best though.

Various states have lots of debt. Japan for example has massive domestic debt – if I understand it correctly, citizens and domestic banks own much of it. Russia and the US, I think, are in a similar situation.

In a sense, that's just some numbers on a spreadsheet somewhere. However, if certain things happened to these numbers, it would be very bad for the economy of these countries. People would get unemployed, production and consumption would decrease.

I'm trying to wrap my head around how to think about the link between this abstract thing – a number somewhere – and a very concrete thing like, Joe the Plumber can't find customers anymore, had to default on his mortgage, and now his family lives in a trailer and survives on beans and ramen.

I do understand the government can't just say, "we're changing this number in the spreadsheet that represents how much we owe to various banks to zero". It would be bad for the economy because it would reflect the government can't be trusted, it also means the bank is worth less now. But I suspect there's some abstract principle behind this that I don't understand, that more directly links "number in spreadsheet" to "guy can't afford tomatoes anymore".

I also know certain governments can in fact partially actually do this (decrease the number in the spreadsheet, or at least its real value), by "printing money" and inflating it away, and that has problems of its own (inflation is bad), but again, I feel like there's some more concrete principle at play here.

Sorry, that's as good as I can articulate this question.