Currency is a medium of exchange...
If it simply represents a barter system of goods exchanged for goods then both sides of a barter are subject to the fluctuations of supply and demand.
No stable medium of exchange is involved.
Which is why a fiat monetary system is preferred if it is kept stable.
Now the problem exists when the fiat money is tampered with by politicians or the wealthy elite or banking systems. These things can destroy a fiat money system...and they don't end well when they do.
The easiest example of explaining this is the petroleum oil market.
Oil price fluctuations are an hourly thing. So is food or lumber or whatever.
Even metals (like gold) are subject to market fluctuations.
Since most oil producing countries export their products they wanted something of stable value for their products.
They originally chose the most stable money which was American dollars.
America, at one time, produced the most petroleum and refined petroleum products.
Now it's others.
And as these foreign countries held more and more US dollars the price of these dollars vx other currencies went exorbitantly high. (Supply vx demand)
So American diplomats talked with OPEC into trading oil contracts using Euros instead of dollars... making American exports of groceries affordable once again.
(USA produces 60% of the world food supply)
The price of oil went sky high as a result for a while... almost $4.00/gal for gasoline.
Today the pressure of fracking the permian basin and other areas and increased supply from OPEC has dropped the price to reasonable prices. The USA also sought to bring out "clean coal" technology which basically turns soft "yellow" coal into diesel fuel that burns cleanly.
Germany developed the process during WW2 but it's been improved since with better tech. We can produce a 55gal barrel of Diesel for @ $75/barrel. Along with the new engines that require DEF... nothing can really do much to control energy prices.
Most countries use american dollars as a secured deposit to back their own currencies they issue...from Australia to Japan... american dollars back up their currencies.