This code provides an example of a custom nav walker class to extend the WordPress Core Walker_Nav_Menu class. The custom nav walker modifies the menu output to use buttons instead of links for menu items with submenus.

Class-Custom-Nav-Walker.php

<?php 
  /*
   *  All code is intended as a learning example only and will require modification to function properly based on the project needs.
   */

  /**
  * This class extends the WordPress Core Nav Walker class to create list output
  * with buttons instead of links for items with submenus.
  *
  * @see https://developer.wordpress.org/reference/classes/walker_nav_menu/
  */
  class Custom_Nav_Walker extends Walker_Nav_Menu {
    /**
    * Starts the element output for the menu.
    *
    * @param string $output Used to append additional content (passed by reference).
    * @param object $item Menu item data object.
    * @param int    $depth Depth of menu item.
    * @param object $args An array of wp_nav_menu() arguments.
    * @param int    $id ID of the current menu item. Default 0.
    * @see https://developer.wordpress.org/reference/classes/walker_nav_menu/start_el/
    */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
      // Wrap each menu item in a list item element with classes and depth level provided from the $item object.
      $output .= "<li class='" . implode( ' ', $item->classes ) . ' menu-item-level-' . $depth . "'>";
      // If the menu item does not have a submenu, output as an anchor element.
      if ( ! ( in_array( 'menu-item-has-children', $item->classes, true ) ) ) {
        // If the menu item is the current page, add the aria-current attribute.
        if ( ( in_array( 'current-menu-item', $item->classes, true ) ) ) {
          $output .= '<a href="' . $item->url . '" aria-current="page">';
        } else {
          // If the menu item is not the current page, output as an anchor element with a URL provided from the $item object.
          $output .= '<a href="' . $item->url . '">';
        }
        // Output the menu item title and close the anchor element.
        $output .= $item->title;
        $output .= '</a>';
      } else {
        /** If the menu item does have a submenu, output the item as a button element 
        * with an aria-haspopup attribute of true and an aria-expanded attribute of false.
        */ 
        $output .= '<button class="megamenu-button" aria-haspopup="true" aria-expanded="false">';
        // Output the menu item title and close the button element.
        $output .= $item->title;
        $output .= '</button>';
      }
    }
  }
?>