who Detect price from Bitcoin to $ or € ?
if i exchange bitcoin into $ and bitcoin exchange it into € will be same (rate between $ and € )
You mean if you exchange your USD to BTC then exchange it to EUR, then will it still be the same value. For example, this is based on preev,
1. 100 USD (85.12 EUR) = 0.0173 BTC
2. 0.0173 BTC = 86.20 EUR (101.31 USD)
That means it does increase when you exchange it USD -> BTC -> EUR. However, this is only theoretically, and it assumes that there are no exchange fees and that you are buying and selling at spot price. So in reality, you will not profit if you did this.