How can the price on a uniswap pair change if one of those tokens is only listed on DEX and not on CEX. How can there be price discovery without arbitrage?

According to what I have read so far prices on uniswap will change because of arbitrage, but without a CEX that lists the token where people are willing to put in buy orders at higher and higher prices … how can there be arbitrage.

And without arbitrage how can the price of a token on uniswap change?