We all know that every address generated on the core wallet or on any other 3rd party wallets like Electrum, Mycellium etc. has a single extended public key (known as xPub) which acts like a master key holding all the addresses like a chain.
Not true. Only some wallets support deterministic keys, and Bitcoin Core is not one of them. Bitcoin Core does not use deterministic keys like Electrum, Mycelium, and Multibit HD do. Not every wallet uses HD (hierarchical deterministic) keys. Wallets like Bitcoin Core still randomly generate all of their keys.
In fact we can find our own xPub from the wallet.
But how exactly can I find that xPub key for any other arbitrary wallets?
I know of walletexplorer.com as it can find the master keys of known companies and exchanges like Coinbase, LocalBitcoins.com, Crypto-Games.net etc. but not an individual's key who is actively using core wallet.
Any kind of help is much appreciated
walletexplorer.com does not know the master keys of companies. It determines wallets by looking for spend links, meaning that multiple addresses are involved in the inputs of a transaction. Since that transaction is able to successfully spend those inputs, then that indicates that the creator(s) of that transaction have access to the necessary private keys. This method is messed up by CoinJoin transactions.
This means that walletexplorer.com does not know the master public key if one for those wallets even exist. They are simply using spend linking. In fact, it is impossible to know the master key of a wallet unless someone tells you what it is. That is the nature of HD wallets.
