£2.1 Trillion Debt Not A Bad Thing Apparently

There was an economist on TV the other day talking about the National Debt and how it had reached £2.1 trillion and the UK Government is currently borrowing a further £30 billion a month which i thought probably explains why Boris Johnson has given up on on the Covid-19 and thrown the doors open and to hell with our health.
I mentioned it to an Economist friend who tried unsuccessfully to explain to me that the debt is not anything to worry about because we are borrowing from ourselves so really no need to rush to pay it back.
Bearing in mind that i spent the whole conversation looking confused and saying 'i don't get it' i may have got it completely wrong but the jist of the argument is that a Government 'borrows' money by writing on a piece of paper (called a bond) that the government promises to buy back on a specific later date and these pieces of paper (bonds) can be bought back with new pieces of paper (new bonds) with later buy-back dates. Our debt then is a good thing because we are investing in our future.
Seeing i was about to say 'WTF are you talking about!' he tried an analogy which went something like this.
You have your £150,000 house with a mortgage but you want to add a new patio or a swimming pool so you borrow the money by borrowing a further £20,000 to pay for the improvements so your debt has gone up to £170,000 but the value of the property is now £200,000 due to the improvements.
The Government is therefore 'borrowing' money in order to make the improvements to the country so we are improving and increasing in value what the future generation will eventually inherit as we can't default on the debt so everyone is a winner and the problem would be if we were not in debt because that would mean the Government was not making the improvments.
It ended with me asking if that's the case, why the Conservative Government under Theresa May and David Cameron gave us a decade of severe Austerity because 'we had to pay off the debt'.
A shrug and a curt 'Because they were Tory w*nk*rs?' was the only thing he said that i understood.
I can't even pretend to understand it and as i said at the start i may have grossly misrepresented what he said but i left none the wiser who we owe £2.1 trillion to and who is stupid enough to lend us £30 million every month.