Generally speaking, no one person or organization.
If the country has a mostly floating (ie not controlled) currency, then the exchange rate is determined by the “market” demand and supply of their currency.
If a country exports a lot, it creates demand for their currency (buyers need to pay for goods in local currency). If a country imports a lot, it creates a supply of currency (ie they exchange local currency for foreign currency).
If a country has a stable and strong economy, does lots of trade, has a reasonable legal/government system and provides a safe location to hold currency assets, international financial institutions will also play a role in the value of the currency.
Central banks can play a “buffer” role by providing supply or demand to the market. This can alleviate short term appreciation or depreciation. Generally providing demand requires the central bank to hold foreign reserves and therefore this is expensive. Providing supply can result in inflation since the money likely circulates back into the domestic economy. So this “buffer” is typically limited unless the government imposes capital controls (too complicated for an ELI5)
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