It is typically exported as a plain text file named MEGA-RECOVERYKEY.txt .
Click your profile image > Settings > Security > Back up key . megaclavederecuperaciontxt+ultima+version+full
Your MEGA recovery key is a generated during the account creation process. Because MEGA does not store your password on its servers, this key acts as a master "backup" for your account's encryption. It is typically exported as a plain text