It takes the main navigation and if there are any submenu items corresponding with the page, it will generate the navigation.

Sub-menu automation of interior menus.php

<?php

function submenu($menu, $returnParent = false) {
  global $post;

  $menu_items   = wp_get_nav_menu_items($menu);
  $menu_childs  = array();
  $menu_IDs     = array();
  $parent_item  = 0;

  foreach ( $menu_items as $item ) {
    $menu_childs[$item->menu_item_parent] = true;
    $menu_IDs[$item->ID] = $item;
  }

  // get current top level menu item id
  foreach ( $menu_items as $item ) {
    if ( $item->object_id == $post->ID ) {
      if(!isset($menu_childs[$item->ID] ) && $item->menu_item_parent==0 ){
        return false;
      }

      $parent_item = ( isset($menu_childs[$item->ID] )) ? $item : $menu_IDs[$item->menu_item_parent];
      break;
    }
  }
  
  // display parent title
  if($returnParent) {
    echo '<h3>' . $parent_item->title . '</h3>';
  }
  
  // display the submenu
  echo '<nav class="sub-menu-wrap">';
    echo "<ul class="sub-menu>";

    foreach ( $menu_items as $item ) {
      if ( $item->menu_item_parent == $parent_item->ID ) {
        $class = ( $item->object_id == $post->ID ) ? "class='current_page_item'" : "";
        echo "<li {$class}><a href='{$item->url}' class='subnav' >{$item->title}</a></li>";
      }
    }

    echo "</ul>";
  echo "</nav>";
}

// To use
echo submenu('Menu Name');