How is interest National Debt calculated?

Hi, I've been messing around with tax receipt tools online to see how much people are paying on national debt.

I've noticed that despite huge changes to the amount owed, the amount of interest paid by the government (in terms of, taxpayer money spent towards debt interest) rarely ever changes.

Is there a limit to what a country pays back each year? How is the interest a country owes calculated each year?

Thank you.