Where can I alter the labels used on the login form?
Try this way by implementing hook_form_alter.



/**
 * Implements hook_form_alter().
 */
function YourCustomModuleName_form_alter(&$form, Drupal\Core\Form\FormStateInterface $form_state, $form_id){
    if ($form_id == 'user_login_form') {
        //- change label for username input
        $form['name']['#title'] = t('Your New UserName Label');
        //- change label for password input
        $form['pass']['#title'] = t('Your New Password Label');
    }
}