Who controls the exchange rate?

I'm curious, who exactly controls the exchange rate of cryptocurrencies? Is it a centralized authority like a government or central bank, or is it determined by the market forces of supply and demand? And if it's the latter, how do fluctuations in the market impact the exchange rate, and what factors contribute to these changes? I'm interested in understanding the mechanics behind how the exchange rate of digital currencies like Bitcoin and Ethereum are determined.