Simply put, a "Wallet" to your digital asset is like a "Bank card" to your fiat asset, while a "Wallet address" is your "Bank account" and "Private Key" is the "Withdrawal password" to your "Bank account". However, not every user holds his or her Private Key to their Wallet.
1. If you have a Wallet in Blockchain or other wallet service providers, you are able to hold your own Private Key;
2. If you are using wallet service in an Exchange such as COLODAX, you do NOT hold your own Private Key.
Why don't I have Private Key to my COLODAX wallet?
COLODAX provides a unique address for each user to deposit cryptocurrency asset to their Colodax account and the address is for deposit ONLY. When users submit a withdrawal request, Colodax will bundle all these requests that are submitted at the same time and send them out from one wallet address altogether. That’s why some of the withdrawals share the same wallet address or TXID.
In this case, the exchange only provides one of the many features in Wallet services - "deposit", making trading more convenient for users, and will not share its Private Keys with users.
How can I have my own Private Key?
Take Bitcoin Cash (BCH) for instance: If you wish to hold your own Private Key, first you need to download a BCH Wallet (such as Blockchain) in your Mobile device or PC. After application, the Wallet will generates a unique BCH wallet and a random string of 256-digit Private Key. For your identification, you need to use Private Key each and every time when withdrawing asset from this wallet.
What is "miner fees"?
In the network of cryptocurrency, every single transaction and its detailed information are all recorded in a "Ledger": Input/output wallet address, amount, time etc. This "Ledger" is known as Blockchain, unique and 100% transparent to all, and its ledger keepers are "Miners". In order to attract miners and speed up the process of transaction confirmation, you will need to pay a certain amount of fees to miners. To guarantee instant confirmation of your transaction, Colodax will calculate and readjust to Best miner fees based real-time crowdedness of blockchain network accordingly. (link: https://colodax.freshdesk.com/support/solutions/articles/47000198946-fees)