How can I design the standard user login form with twig?
I found the solution how to add user and password placeholders an modify user and password labels:

function THEMENAME_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'user_login_form' ) {
    // Add placeholders.
    $form['name']['#attributes']['placeholder'] = t('User Placeholder Text Here');
    $form['pass']['#attributes']['placeholder'] = t('Password Placeholder Text Here');

    // Labels
    $form['name']['#title'] = t('User Label Here');
    $form['pass']['#title'] = t('Password Label Here');

    //Submit Value
    $form['actions']['submit']['#value'] = t('Submit Button Text Here');
  }
}


To remove "Create new account" and "Reset your password" links, just override block--user-login-block.html.twig file. Change 

{{ content }} 


to

{{ content|without('user_links') }}