The same way the price of a banana is determined. Supply and demand: if more people want to buy bananas than there are bananas the price increases. If you have a lot of bananas but not many want to buy them you’ll lower your price until someone says “that’s a good deal for a banana!”
In very few cases the central bank will attempt to regulate the price of a currency, mainly when pegging the value to a different currency. However, for the most part it’s mainly just what people are willing to pay for it.
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