The value changes because fundamentals change. Stock value rises because a company is earning more; the USD rises against the JPY because the US economy is doing better (both very simplified examples).
The price, which is different from value and is what you see fluctuating day to day, changes because there was a buyer and a seller that transacted at that time. Someone thought the value of Nvidia would increase so they paid price x, and someone sold it at price x because they didn’t have the same view/any other reason. This happens over and over again with different market participants for different reasons at different prices – hence the fluctuations.
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