How to get related posts and wp comments under tabs

You need to use do_shortcode() for the shortcode to actually work inside PHP

 function add_post_content($content) {
    if(!is_feed() && !is_home()) { 

    $above="<p></p>";
    $content_of_first_tab = 'A list of related posts ';


    $args= array(
        'number'  => '5',
        'post_id' => get_the_ID(), // use post_id, not post_ID.
         );
    $comments = get_comments( $args );
    $output="";
    foreach ( $comments as $comment ) :
        $output .= $comment->comment_author . '<br />' . $comment->comment_content. '<br />';
    endforeach;
    $content_of_second_tab = $output ;
    $below = do_shortcode( '[tabby title="First Tab"]' . $content_of_first_tab . '[tabby title="Second Tab"]'. $content_of_second_tab. '[tabbyending]'  );

    $content = $above.''.$content .''.$below;
    }
return $content;
}
add_filter('the_content', 'add_post_content');

This question is specific to a plugin and should be posted in the appropriate support forum

Do not copy and paste code unless you know what you’re doing.

I hope that helps you.