If I am trading from the order book, on say Bittrex and I am looking at the volume and the support walls etc... , am I looking at only the trades from that particular exchange or am I looking at the trades/walls etc from all the exchanges put together?
As TryNinja said, Bittrex or any other exchanges only show sell and buy orders submitted on it, not other exchanges. It's impossible to have all of orders in different exchanges in a single wall.
If I am only looking at the data from that particular exchange, how can anyone make an informed decision when there are a multitude of other things happening on other exchanges?
Prices on different exchanges are very close to each other.
Bots and arbitrage traders don't allow the price on different exchanges to be much different.
So, if you are trading in an exchange with a sufficient volume, don't worry about other exchanges. Prices are always same.
Surely, the data must be an aggregate of all the exchanges like that displayed by CMC.
Coinmarketcap extract the price from different exchanges and calculates the weighted average price for every coin.