PHP - Need Help Adding Dynamic Include Expression

I am building a site using PHP, OOP, and an MVC framework. i am not using laravel, codeignitor, or any other framework, I am building this on my own. I am a newbie to mvc and so I am learning how it all works.

I have a dynamic nav menu that populates the categories from the db. The menu/categories will populate and work fine when I view the nav page on its own, but it will not work when it is included in the index view page. What should I do to make the dynamic menu view on all the view pages that i need it to? 

Below is my code, any guidance would be greatly appreciated.

Controller:

class Behindthescenes extends Controller
{
    public function __construct()
    {
        $this->btsModel = $this->model('mBehindthescene');
    }

    public function index()
    {
        $data = [
            'title' => 'Admin Dashboard',
        ];
        $this->view('/behindthescenes/index', $data);
    }

// Get categories for nav menu
    public function bts_cat_nav()
    {
        $catnav = $this->btsModel->getCatsNav();
        $data = [
            'catnav' => $catnav,
        ];
        $this->view('/includes/bts_cat_nav', $data);
    }
}

Model:

class mBehindthescene
{
    private $db;

    public function __construct()
    {
        $this->db = new DBC;
    }

    // Get Categories for Admin Navigation
    public function getCatsNav()
    {
        $this->db->query('SELECT * FROM categories ORDER BY cat_name ASC');
        $result = $this->db->fetchMultiple();
        return $result;
    }
}

Index View:

require_once APPROOT . '/views/includes/header.php';
require_once APPROOT . '/views/includes/bts_nav.php';
require_once APPROOT . '/views/includes/bts_cat_nav.php';

?>

<div class="container-fluid">
  ADMIN DASHBOARD

</div>

Nav Menu View:

<div class="container">
  <div class="row">
    <div class="cat_nav">
      <ul>
        <?php foreach ($data['catnav'] as $cat): ?>

        <li class="nav-item"><a href="<?php echo $cat->cat_link; ?> class=" nav-link text-white p-1 mb-1
            sidebar-link"><i
              class="<?php echo $cat->cat_fa; ?> text-light fa-lg mr-3"></i><?php echo $cat->cat_name; ?></a></li>

        <?php endforeach;?>
      </ul>
    </div>
  </div>
</div>