“What, exactly, “backs up” the US dollar other than the full-faith-and-credit of a corrupt US feral government . . .”
The authority to tax wealth and the power of state violence to enforce that authority. The government can not only print money it can take anything from you it wants to take.
I’m not suggesting that you like that answer; but that is the answer.
Evidently you are too dim to be aware of the Weimar Republic level of inflation that is about to devour us all.
Good luck to ya.