Who determines the value of BTC vs USD? How is the data collected to determine Bitcoin's value? And who determines the exchange value of Bitcoin in USD?

This question isn't really answered, no matter how much I research. I want to know the evaluation process of Bitcoin. If possible can someone go as much detail as they think necessary. I understand the mechanisms of Bitcoin: generation, exchange and the what not, but never really understood how or whom values Bitcoin.

I understand that currency is evaluated by demand and supply but how is the demand and supply gauged and who does this seemingly impossible task? Because, you know, Bitcoin is supposed to be unregulated and free.
The Bitcoin / USD exchange rate is determined by users of an exchange. Traders in an exchange can buy/sell at however much they'd like, but if you sell BTC to USD at too high a rate, or buy BTC with USD at too low a rate, your transaction is less likely to occur.