Op if I can memorize my seed phrase or the private key and know it the way I know my mobile phone digits
It is harder to remember your wallet's seed phrase or private key than remember your mobile phone number.
then I would be the happiest person on earth because at that I don't need to save my phrase in different places again but probably in one of particular place because even at that I might forget some characters.
Happiest person only if you don't forget it and lose your wallet and bitcoins.
Your memory can be lost, your brain can be damaged by physical attacks, accidents, health problems and this method is very risky one.
How to back up a seed phrase?If not, I know my phrase i know my password for off... Then that should be a plus to me for the safety and security of the wallet.
Your password is a second layer to block access to your wallet. If you forget your seed phrase, are you sure you will remember your wallet password?
When something happens with your brain, memory, you can forget all of them, wallet seed phrase, private key, wallet password.