The same way today prices are determined, by free market determining price. Does it really matter if the price for breed in your local store is in USD, EUR or Bitcoin? No, the price is found the same way.
No, the Eur derives its value largely off the dollar, the dollar is aka The petrodollar, once you have secured your dollar amount per barrel, this commodity trading, largely driven by speculation, will then determine the price of everything else.
It was something 1-3 years ago during the time the US was really pressing hard on iran with sanctions and basically cut off their access to the rothchild banking system, boom, petrolgold was reignited causing the US sanctions to back peddle to keep them on the dollar system.
Iran was trading its oil for gold. The powers that be did not like this.
However gold still derives its value from the dollar, for the most part.