What they stored in cold wallet? All bitcoins anyone owned is already in blockchain I just want to understand what actually they store in the cold wallet because all bitcoins anyone owned is already in the blockchain.

Do they store the private key of users in cold wallet?
Do they store the private key of users in cold wallet?


In short, yes. (Some) private keys are stored in cold wallets.

The record of all bitcoins (UTXOs) exist as a public database, which is stored in full nodes all around the world.

To spend a UTXO (ie, some amount of bitcoin), the user must be able to produce a valid signature, by using the appropriate private key for that bitcoin address/script. 

Those private keys are stored in various ways. One such way is a 'cold wallet'. Generally, a cold wallet describes a way of storing a bitcoin private key, such that the device storing the private key has not, and will not, connect to the internet. Conversely, a wallet that exists on an online machine is usually referred to as a 'hot wallet'. 

There are also different types of hot wallets, and cold wallets. But the online/offline distinction is the main distinguishing factor.