Drupal 8: Prevent node pages (full view) being shown to non-admin users.

module.php

<?php

use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

/**
 * Implements hook_preprocess_HOOK().
 */
function my_module_preprocess_node(&$variables) {
  $node = $variables['node'];
  $nodetype = $node->getType();
  
  // Only allow pages to be viewed by non-admin users:
  if ($variables['view_mode'] === 'full' && !$variables['is_admin']) {
    $legal = [
      'page',
      'node_type',
    ];
    if (!in_array($nodetype, $legal)) {
      throw new NotFoundHttpException();
    }
  }
}