You can create and access your account either with an email + password, or with the "Sign in with Google" button, whichever you prefer, both lead to the same kind of account with the same features.
What happens if you use both
If you sign up with Google using the same email address as an existing password-based account, CodeBlock Studio links them into a single account rather than creating a separate one. From that point, either method signs you into the exact same account, same projects, same settings.
Why some actions still ask for a password
If a password was ever set on your account (even if you now mostly sign in with Google), certain sensitive actions, most notably deleting your account, still ask for that password as an extra confirmation step. This is because Google sign-in itself doesn't involve a password at all, so it can't be used as that same kind of proof, the password check exists specifically as a deliberate, separate confirmation.
If you don't remember that password, use Forgot Password to set a new one, it works the same way regardless of which method you normally use to actually sign in day to day.