How does coinmarketcap see who has how much of the market? In other words what information exists in the blockchain? Is there a link to which someone could point me to as to whether we can get this information from the block chain
This information does not exist on the blockchain. It is gathered from exchanges. The blockchain has no idea what fiat is nor does it care. The only information that exists on the blockchain that sites like coinmarketcap uses is the total number of coins in existence which is then used to calculate the market cap. Everything else is from exchanges.