I wondered this as most countries have had to throw humongous amounts of cash at covid, so who would have any spare to lend to someone else?
The answer was that we are borrowing from ourselves, or perhaps to be more accurate, from our future self's. The government issues bonds which it promises to repay plus interest in "x" years and they are purchased in the main by pension funds and financial institutions.
When the time for repayment is up, the government (unless it has found other revenue) will issue more bonds to meet the liability which are promptly bought by the same investors using the money that they received from the maturity of the previous bond issue.
While everyone keeps playing ball, it can roll on indefinitely, can't it........