D8 : Load menu entries from menu and menu_root

D8LoadMenuEntries.php

<?php

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Url;

class Controller extends ControllerBase {

  function loadAdminMenuEntries($menu_root, $menu_name = 'admin') {
    $menu_items = [];

    $parameters = \Drupal::menuTree()
      ->getCurrentRouteMenuTreeParameters($menu_name)->setRoot($menu_root);

    $tree = \Drupal::menuTree()->load($menu_name, $parameters);

    if ($tree[$menu_root]->hasChildren) {
      $subtree = $tree[$menu_root]->subtree;

      foreach ($subtree as $item) {
        $menu_items[] = [
          'title' => $item->link->getTitle(),
          'description' => $item->link->getDescription(),
          'url' => $item->link->getUrlObject()->toString(),
        ];
      }
    }

    return $menu_items;
  }
}