Changing the Default New User Notification Email

Try it with a filter:

add_filter( 'wp_new_user_notification_email', 'custom_wp_new_user_notification_email', 10, 3 );

function custom_wp_new_user_notification_email( $wp_new_user_notification_email, $user, $blogname ) {
   // filter stuff in $wp_new_user_notification_email here
   return $wp_new_user_notification_email;
}

Source: https://developer.wordpress.org/reference/functions/wp_new_user_notification/#comment-3130