comments.php fallback?

I am trying to customize my comments.php and i literally copied an pasted the comments.php file from twenty fifteen. However i dont understand how this code works. The code only implies an ordered list, but when i view it in chrome it has listitems, gravatars and alot of classes etc. So where do those classes comming from? I want to customize it to my liking.

<ol class="commentlist">
     <?php
     wp_list_comments(array(
      // see http://codex.ww.wp.xz.cn/Function_Reference/wp_list_comments
      // 'login_text'        => 'Login to reply',
      // 'callback'          => null,
      // 'end-callback'      => null,
      // 'type'              => 'all',
      'avatar_size'       => 50
      // 'reverse_top_level' => null,
      // 'reverse_children'  =>
      ));
      ?>
    </ol>