Well if you have a poleniex account and want to withdraw you can check their features on how you can withdraw them. There are many different ways in each different websites but most of them you need to get your wallet address and put it in when you trying to withdraw now but if your trying to say is that when you have a different coin and not bitcoin you actually need to exchange it into a bitcoin you need to go to the coin that you have and then sell it into a bitcoin and then you can withdraw it but some of the sites have fees and or specific amount on the minimum BTC you can withdraw. So you need to check that first before you need to withdraw. Hope it helps