Bitcoin is constantly being traded on a number of exchanges, on these platforms there is;
• Demand - how much people are willing to pay for it, and
• Supply - how much traders are willing to sell it for
The price is calculated based on these factors, the higher the damand against supply, the higher the price.

There are different order books on the different exchanges, reason why there is no fixed price. The value on binanace would be different to the one on coinbase.
Cointrackers like coingecko and CMC calculated the average price of various exchanges.