The US government can issue more fiat to repay bonds if necessary. It can literally print itself out of debt. Yes, this could potentially cause some devaluation, but it's still a powerful backstop. The US would only default by choice, rather than being forced to. This is the privilege of having the USD be the world's reserve currency.
At the level of the world's biggest economies, the concepts of debt (bonds), money, and interest rates are more abstract economic levers that central banks pull in order to keep economies chugging. They're not really the same things they are to individual people and corporations, e.g. in the way debt can cripple someone's finances.
The central banks and governments are the game masters who set the rules and issue tokens, and we're the players interacting within the game and responding to the environments that they create.
No, the US can simply create more USD and use it to pay off bond debt. Creation of USD doesn't incur more debt, but does expand the USD monetary base and inject more USD liquidity into global markets. The US doesn't actually need to borrow more to pay off old debt.
To restate: bonds and other loans are literally a policy tool to take in USD from or inject USD into circulation, in order to intentionally change the size of the money supply: "Open market operations (OMO) refers to a central bank buying or selling short-term Treasuries and other securities in the open market in order to influence the money supply." It's not like the central bank actually needs to borrow USD from the money supply; it's literally the issuer of USD and could create more at any time (or take it in and destroy it).
Why can the US do this? Because its bonds are denominated in its own currency, which world investors use. The demand for USD-denominated debt is a major benefit of the USD being the world's main reserve currency.
At the level of the world's biggest economies, the concepts of debt (bonds), money, and interest rates are more abstract economic levers that central banks pull in order to keep economies chugging. They're not really the same things they are to individual people and corporations, e.g. in the way debt can cripple someone's finances.
The central banks and governments are the game masters who set the rules and issue tokens, and we're the players interacting within the game and responding to the environments that they create.