You can change the source of the price for each coin in your Portfolio or Watchlist.
Open the coin detail screen and above the charts, to the right, you will see the source exchange and pair displayed. You can tap it to select the exchange and trading pair you'd like to view prices from. The chart will also update to reflect the new source.
Global average is calculated by Delta based on prices from the different exchanges we support.
Take the example of Ethereum and the source selected is Binance ETH/BTC. The app will show the ETH/BTC graph with prices sourced from Binance. The price is converted using the exchange's BTC/USD or BTC/USDT rate to display the price per coin in USD (or your local fiat currency). If the exchange selected does not have a BTC/USD(T) pair then the BTC/USD value from global average is used. You can tap on the ETH price in USD to interconvert between USD and the pair selected (BTC in this case).