Get page IDs for certain kinds of pages

page-ids.php

<?php

// Special pages -----------------
$frontpage_id = get_option( 'page_on_front' );
$blog_id = get_option( 'page_for_posts' );

// Page templates -----------------
$about_templates = get_pages(array(
	'meta_key' => '_wp_page_template',
	'meta_value' => 'page-about.php'
));
if($about_templates){
  $about_id = $about_templates[0]->ID;
  $about_url = get_permalink($about_id);
}
else{
  $about_url = '#';
}

// OR:
$args = [
    'post_type' => 'page',
    'fields' => 'ids',
    'nopaging' => true,
    'meta_key' => '_wp_page_template',
    'meta_value' => 'page-about.php'
];
$about_templates = get_posts( $args );
if($about_templates){
  $about_id = $about_templates[0]->ID;
  $about_url = get_permalink($about_id);
}
else{
  $about_url = '#';
}

// Privacy Policy Page -----------------
get_privacy_policy_url(); // Retrieves the URL to the privacy policy page.
the_privacy_policy_link(); // Displays the privacy policy link with formatting, when applicable.
get_the_privacy_policy_link(); // Returns the privacy policy link with formatting, when applicable.
?>

<!-- Usage: -->
<a href="<?php echo $about_url; ?>">About Us</a>