The traditional way a state defaults is when their debts are in a currency they have to exchange with their own currency on the open market. Vs their own which they can create via central bank actions. In the former a self-sustaining collapse in exchange rates makes it impossible for the state to buy enough foreign currency to make their debt payments. In the former case, default happens, see Argentina in 2001 or so. In the latter this doesn't happen, see Japan 1989-present.

In the US, all of Federal and state governments debt is denominated in dollars. Varying exchange rates don't effect the ability of the government to pay debt. And the government can borrow money from the Fed, because it can.