How do I render search block form in theme template?
Here are the 3 methods I found.
1 - Use plugin.manger.block to render search_form_block
-- in mytheme.theme
function MYTHEME_preprocess_page(&amp;$variables) {
  $customblock = \Drupal::service('plugin.manager.block')-&gt;createInstance('search_form_block', []);
  $variables['content']['custom_block_output'] = $customblock-&gt;build();
}

-- in page.html.twig
{{ content.custom_block_output }}

2 - Same as 1 but render a customized search block from block--search-form-block.html.twig.
-- in mytheme.theme - Same as method 1
-- in page.html.twig
{% include '@MYTHEME/block--search-form-block.html.twig' %}

3 - Use Twig Tweak - credit to No Sssweat's Post

install the Twig Tweak module
insert {{ drupal_block(search_form_block') }} on page.html.twig