Writing the iframe tag at the top of your html.html.twig file in Drupal 8/9

tagmanager.php

<?php
function tagmanager_page_top(array &$page_top) {
  if (FALSE === \Drupal::service('router.admin_context')->isAdminRoute()) {
      $page_top['above'] = [
        '#noscript' => true,
        '#type' => 'html_tag',
        '#tag' => 'iframe',
        '#attributes' => [
          'src' => sprintf('https://www.googletagmanager.com/ns.html?id=%s',$settings->get('gtm_code')),
          'height' => 0,
          'width' => 0,
          'style' => "display:none;visibility:hidden"
        ]
      ];
  }
}