Step 1. Create a custom module and add the HOOK hook_views_data_alter(&$data). Here module : mymodule

Step 2.

You have several mthodes to override 403 / 404 System pages.

Method 1. Set 403 / 404 pages.

You can set the 403 /404 page on "Basic site settings" page
Admin -> Configuration -> System -> Basic site settings
Path /admin/config/system/site-information

An email is classified as spam or not spam on the receiving end, not the sending end. But you can improve

Example :

$result =  array(
  '#markup' => 'Hello. This is my First Page',
);
$renderer = \Drupal::service('renderer');
$html = $renderer->render($result);

This tutorial show toy how to insert an existing views into a custom page or custom block.