At the expiration of the contract you are trading, all positions get closed out and settled. Different exchanges use different anchors to settle against. To avoid manipulation, most of them use an index. OKCoin uses a custom 6-exchange index (3 chinese, 3 Western), while BitMEX, Deribit, Coinpit, and CryptoFacilities use a multi-spot-exchange Index to settle all of its contracts at expiration. This minimizes the risk of any manipulation if contracts settled on a single exchange's price, but it also makes it more difficult to manage your hedges since there's no way to "buy" or "sell" an index. No system is perfect, and each one offers different unique pro's and con's, just like your selection of contract type and length. So depending on your case you may want something different than someone else.
There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.