If you notice an unfamiliar username on your account, it was most likely generated when you used a Sign up button > “Sign in with Apple”option. In this case, the username is automatically assigned and cannot be changed through the standard settings.
Apple’s password manager automatically generates strong, random usernames and passwords to improve security. These credentials often appear as random alphanumeric usernames for apps and websites. These are generated to prevent password reuse and ensure that each account has unique, secure credentials via Apple Privacy Policy.
You can read more about “Hide my email option” here.
If you want to change your username, please follow this article.