display Root Category in top menu nav

Question

Hello I want to display a "Default Category" in nav main menu and all subcategory inside this. I know this question has been solved but I do not know how to do that

i have a template in vendor topmenu.html

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

/**
 * Top menu for store
 *
 * @var $block \Magento\Theme\Block\Html\Topmenu
 */

$columnsLimit = $block->getColumnsLimit() ?: 0;
$_menuHtml = $block->getHtml('level-top', 'submenu', $columnsLimit)
?>

<nav class="navigation" data-action="navigation">
    <ul data-mage-init='{"menu":{"responsive":true, "expanded":true, "position":{"my":"left top","at":"left bottom"}}}'>
        <?= /* @noEscape */ $_menuHtml?>
        <?= $block->getChildHtml() ?>
    </ul>
</nav>

I see this solution but i don't know to add this because i have a error when i add this code in override topmenu Error: Class 'Mage' not found

  <?php

  $root_category = Mage::getModel('catalog/category')->load(3); // Put your root category ID here.
  $subcategories = $root_category->getChildren();
  foreach(explode(',',$subcategories) as $subcategory) {
        $category = Mage::getModel('catalog/category')->load($subcategory);
        echo '<a href="'.$category->getURL() .'" />'.$category->getName().'</a><br/>';
  }

  ?>

Please how to do to display root category and submenu in main menu

thanks

Answer 1

Magento2 does not use Mage class,

You should use category factory object (Magento\Catalog\Model\ResourceModel\Category\CollectionFactory) in block constructor

$objectManager = $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$categoryFactory = $objectManager->create('Magento\Catalog\Model\ResourceModel\Category\CollectionFactory');
$categories = $categoryFactory->create()                              
    ->addAttributeToSelect('*')
    //->setStore($this->_storeManager->getStore())
      ; 

foreach ($categories as $category){
    $category->getName();
}

Reference How to get Category Collection per Store in Magento 2?

https://www.rakeshjesadiya.com/how-to-get-category-collection-by-category-id-in-magento-2/