How to embed a node's NID in the title of a node? I know the Automatic Nodetitles module exists, but it is only for Drupal 7. 

Is there a method of automatically assigning the title with the NID upon clicking submit? I know that the NID doesn't exist until the submit button is hit. How would one embed the NID in the title? 

What hook would one use?
You need to do two step:

Step one: auto generate the title and hidden form. use hook_field_widget_form_alter() with code

function MY_MODULE_field_widget_form_alter(&amp;$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
  $field_definition = $context['items']-&gt;getFieldDefinition();
  $bundle = $form_state-&gt;getFormObject()-&gt;getEntity()-&gt;bundle();
  // You can check with other bundle
  if ($field_definition-&gt;getName() == 'title' &amp;&amp; $bundle == 'page' &amp;&amp; $element['value']['#default_value'] == '') {
    $element['#required'] = FALSE;
    //hidden title  
    $element['#type'] = 'hidden';
    $element['value']['#required'] = FALSE;
    // Set default value
    $element['value']['#default_value'] = 'Default Title';
  }
}


with step one you can cross submit form.

Step two: Override new title with default value. Use hook_node_insert()

function MY_MODULE_node_insert(Drupal\node\NodeInterface $node) {
  if ($node-&gt;bundle() == 'page') {
    $new_title = 'New title ' . $node-&gt;id();
    $node-&gt;setTitle($new_title);
    $node-&gt;setNewRevision(FALSE);
    $node-&gt;save();  
  }
}