Who determines how much your crypto is worth?
Does uniswap do this? No, not on their own. They can't --- their daily trading volume is too small. They are literally forced to follow the "marketplace".
The 800lb gorillas that make up the crypto "marketplace" are the centralized exchanges --- Binance and others. They set the price using whatever method they choose. And they can easily influence the price by buying and selling themselves. Binance has access to unlimited funds to do so --- by simply minting more Tethers --- which they have done repeatedly --- and the price remains pegged at $1USD (wink, wink, nod, nod).
With unlimited funds, effective control over the pricing mechanism and no regulation --- the centralized exchanges have all the ingredients for the ultimate form of insider trading fraud.
If you trade crypto as it exists today, you are *trusting* the centralized exchanges who have overwhelming influence over the "marketplace". You have no other option.