That password protected wallet file remains password protected, even if you import that into a new electrum wallet ?
No, the password you set up only encrypts the wallet locally, if you or an attacker imports this wallet with the seed phrase on a different device, you'll have access to spend the funds on the wallet and set up a different password for the wallet on that device.
That's interesting. So as we say that the password protected wallet is encrypted, it is encrypted only if it remains on that particular system. If someone copies that wallet file generated by the electrum and imports it into another computer, it will not remain encrypted???
No, that is not correct. When you encrypt the wallet data container (file) on one computer, it remains encrypted even after importing (opening) it with the software on another computer. Of course, you will need a password to access it, and that is the whole point. However, if you recreate the same wallet from a backup seed phrase, you will once again have the option of whether you want to password protect (encrypt) the wallet file or not because it will be a new wallet file.
You do not have to encrypt the seed phrase because you can simply create a BIP39 passphrase. In this case, the seed phrase itself is useless without the additional passphrase, which you can write down and store in a safe location separately from the seed phrase.
So we can write a seed phrase on a piece of paper and make such a passphrase that we never forget, and save that passphrase in our mind. So now if anyone gets our seed or if the seed is somehow get hacked through malware or something, the hacker won't be able to get access to our wallet.
The BIP39 passphrase is designed to protect your backup seed phrase. However, it is not encryption; it is simply the method for generating wallets from a seed phrase. For instance, you can generate a wallet with a specific 24-word seed phrase. But if you add a passphrase to that same seed phrase, you will create an entirely new wallet with a completely different set of private keys and addresses. Think of it as the 25th word (although it doesn't have to be a dictionary word, and it doesn't have to be just one word since it is a passphrase, not a password).
However, if the hacker gets the whole wallet file (encrypted one), then he can import it into another computer and can access the wallet ?
No, encrypted wallet files cannot be opened on any computer without the correct password. They can potentially be brute-forced if the password is too weak.