FRNs derive their status as *money* from government edict, they derive their *value* from their value of the recent past. If you keep backing up you will get to a point where FRNs derived their value based on gold or silver backing - commodities that have actual value. That is the gist of the regression theorem, and why determining the purchasing power of bitcoin is a seemingly impossible task. Fiat by government force or fiat by voluntary coalition, fiat is fiat because someone says it's so and not because of any real value.
Gold and silver are not identical. Gold is more scarce, has higher industrial use value, and higher subjective value in general.