The related posts code is located in templates/related_posts

In there we create the similar query by using the tags of the currents posts
'tag__in' => array($first_tag),

The actual query is made at line 28
$my_query = new WP_Query($args);

while the blog list display is made via

get_template_part('templates/related_post_unit');

Please note that for related posts we use this file related_post_unit.php and not the blog_unit.php or blocg_unit2.php like on regular blog list.