Bitcoin's price is decided the latest executed buy/sell offer on exchanges. Let's say 5 seconds ago someone bought 1 BTC for a price of $36,000 on Binance. Bitcoin's price on Binance would then be $36,000 (temporarily, until the next order executes, which should only take mere seconds).

Now, on a bigger scale(e.g. CoinMarketCap), Bitcoin's price listed there would be the average price of all the exchanges listed on CoinMarketCap.

Also, supply and demand. Let's say you live in a country where people totally love apples so so much where people are frequently buying them any time of the month. And since farmers are totally aware of the people love for apples, a lot of farmers plant them. Now, a huge ass storm came, destroying like 90% of the apple plantations in your country. Since the demand of apples stayed high while the supply dropped significantly because of the storm, the price would then rise a good chunk.