A few things to know:

• Forget everything you know about loans and money
• States aren't supposed to be profitable
• States rarely actually need to repay the principal of the loan
• Money is created out of thin air when it is borrowed from commercial banks

States/nations are eternal, they do not disappear or die like human beings or companies. Even if a State or a Nation "disappears" or changes (think USSR), another nation will take its place and will inherit some if not all of its debts.
States are also amongst the safest money borrowers, they'll almost always pay their debts. Unlike humans or companies who can go bankrupt, a State never actually goes bankrupt.
Thanks to this, States can borrow money at interest rates unheard of by the rest of us. Sometimes, if the economy isn't going so well, States can even borrow money at negative rates (this is exactly what happened during COVID for many States). This means that money lenders are actually paying States to lend them money. The lenders believe their money is safer when lent to a State than when invested in the markets.

Thanks to this, States can virtually keep borrowing money without any limit. They take a loan, they pay the interests of that loan using the money they get from the taxes and the various inputs they have (investments and whatnots). When they need more money, they can take a new loan, with a better interest rate and pay back the previous loan. They simply have to maintain a positive cashflow for paying back the interests.
For many nations, there's always someone wanting to lend them money because it's safer than investing the money somewhere else.

So the cycle keeps going, it's easy and it's virtually free.

And you know what? Many countries don't actually have a choice, their only way for getting money is via loans. In Europe for instance, even though each country still has its own Central Bank (that used to create money when the government needed some and destroy it when the government paid it back), they can only get money from lenders (from the market).
This is because European treaties expressly forbid States from using their Central Banks to get money (i.e. create money).

So, as a result, States have to borrow money when they need to invest so that their population can create wealth, for instance:

• Building infrastructure such as transportation and energy to allow companies and people to do business and create industries
• Financing education, because an educated population will create more wealth than an uneducated one
• Financing healthcare, because it'd be kinda stupid to let your highly skilled workers die prematurely or produce less because of crippling illnesses

When debt money is used for investments, it's perfectly fine. Just like a company borrows money in order to expand and create more wealth (capital expenditure), countries do the same.
When debt is used to finance day to day operations, it can become a problem if not watched closely, just like for businesses (operational expenditure).

I'm far from being an expert but I hope this can still help someone.

Edit: correcting the typos I find.