Based on the answer HERE
The following is possible
entropy --> random number --> phrase --> private key --> public key --> address | ^ | '------------------------' +--> private key --> pubkey --> addr +--> private key --> pubkey --> addr : '--> private key --> pubkey --> addr
Because HD wallets generate a new address for each transaction, behind the scenes they are using the fixed derivation path to generate new private keys and public keys from which to create the address.
many other private keys are generated from this using a "derivation path" which can differ between different brands of wallet. The derivation path is chosen by the developers and some different developers chose different derivation paths.
So my question how do i generate a private key from another private key or derivation path like stated in the answer of that post?