I mean that in a monetary economy, one always buys goods and services with money. One side of the transaction is money, whatever it is. Sophisticated here means an economy that has progressed beyond barter and developed a medium of exchange and a store of value, ie, Money.

Important to note separately that multiple currencies are results of legal tender laws. In the absence of such interference, the market would choose one commodity as Money (refer the linked article) and it would not be paper with ink on it. Yes, what could happen is that the chosen commodity is then represented by pieces of paper, but they are simply a more convenient token for the commodity, which is kept in store. And there could be multiple different papers with different names, but they all refer to the stored commodity and are thus economically equivalent and with fixed ratios among them. This economically sound system used to exist - close enough - in the 19th century.

Even now, the various currencies are not different in any meaningful economic sense. Non-US countries use the USD as the reserve for their own currencies. Some also use the Euro, Yen etc. But analyze the Euro and one sees tender laws and gold standing behind it. What stands behind the USD? Laws and legitimacy of the govt enforcing those laws. There is gold, which would be called upon if peple reject the currency as it is. We need to separate the surface phenomena from the economic relationships underlying them.