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Imagine creating your wallet like throwing with multiple dices.
It will generate some random number, that’s your address.
Adding a passphrase is like adding some numbers to those generated by dices. So it won’t be the same address (number).
It’s not like a password where you can have your PC unlocked and they set a password and keep all the data in the place.
With one seed you can have those addresses:
Address 1 (without passphrase)
Address 2 (with some passphrase)
Address 3 (with other passphrases)
Address 4 (with other…)
*it should be called wallets but I’m too lazy to rewrite it, the address is something different
You won’t lose bitcoins if you create another wallet (they don’t disappear), but you won’t be able to move them (make transactions) unless you know the seed AND passphrase (if set some).
Unless a password, nobody can help you reset it (look back at the dice example, if you add some number, how could anyone know that number?)
More here (feel free to use other sources, they will probably exist in your language too if it differs from English)
https://learnmeabitcoin.com/technical/mnemonic