How do I render a  tag?
Probably iframe is filtered in #markup. Use an inline_template instead:

  'example one' =&gt; [
    '#type' =&gt; 'inline_template',
    '#template' =&gt; '&lt;iframe src="{{ url }}"&gt;&lt;/iframe&gt;',
    '#context' =&gt; [
      'url' =&gt; 'www.google.com',
    ],
  ],