Bitcoin value is determined by the buyers and sellers. Read about supply and demand and you will know in detail. In summary, the seller sell BTC at a price level and the buyer thinks it is worth the price and buy it.
Yeah this is the main thing that bitcoin will have the value, so for people who want to understand that, they should read about supply and demand like you said. Now the demand of bitcoin is too high so we see that bitcoin price increase too much.