I created a simple test module that adds a text banner to all nodes if a checkbox is checked in the form of the modules configuration. Currently it is working halfway as I want.

When the checkbox is checked and the configuration is exported the text banner displays on all nodes.

So far this is correct. But I don't want to have to export the configuration for the text banner to display. I want it to display immediately after checking the checkbox.

An example is in Devel. If you go to the admin area of Devel and check the box 'Display $page array' The page array displays on every node afterwards without having to export the configuration

To display a text banner on all nodes I am using hook_node_view. Then getting the checkbox value from my custom module settings.yml file.

function my_form_configy_node_view(array &$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {

  $config = \Drupal::config('my_form_configy.settings')->get('checkbox');

  if ($config === TRUE) {

      $build['layout_test'] = [
        '#markup' => '<h2> Extra, Extra read all about it.</h2>',
      ];
    }
}

This is my form

namespace Drupal\my_form_configy\Form;

use Drupal\Core\Database\Database;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Form\ConfigFormBase;

class MyForm extends ConfigFormBase {

  public function getFormId() {
    return 'my_form_configy_1346';
  }

  const SETTINGS = 'my_form_configy.settings';

  protected function getEditableConfigNames() {
    return [
      static::SETTINGS,
    ];
  }


  public function buildForm(array $form, FormStateInterface $form_state) {

  $config = $this->config(static::SETTINGS);

    $check = \Drupal::config('my_form_configy.settings')->get('checkbox');

    $form['checkbox'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Do you want to add the text banner to all nodes? Checking this box will add a large h2 text banner to add nodes. You must export the config and reload a node page'),
      '#default_value' => $check,
      '#weight' => '0',
    ];

    $form['#theme'] = 'my_form';
      return parent::buildForm($form, $form_state);
  }


  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->configFactory->getEditable(static::SETTINGS)
      ->set('checkbox', $form_state->getValue('checkbox'))
      ->save();
    parent::submitForm($form, $form_state);
  }


}

Devel is using the same method I am, but if you check the box 'Display $page array' the dpm feature works on nodes immediately without having to export the configuration.

function devel_page_attachments_alter(&$page) {
  if (\Drupal::currentUser()->hasPermission('access devel information') && \Drupal::config('devel.settings')->get('page_alter')) {
    dpm($page, 'page');
  }
}

I have searched for ConfigFormBase in core and contrib modules and read this. https://www.drupal.org/docs/drupal-apis/configuration-api/working-with-configuration-forms and https://www.drupal.org/docs/8/api/form-api/configformbase-with-simple-configuration-api

Update. 4k4 comment below put me in the right place. According to this post https://drupal.stackexchange.com/a/266393/58880. I needed to apply addCacheableDependency

\Drupal::service('renderer')->addCacheableDependency($variables, $config);

Since I am using hook_node_view. I changed $variables to $build

\Drupal::service('renderer')->addCacheableDependency($build, $config);

Now when I click the checkbox the value of chechbox is correct and its not coming form the config. The text banner is displayed on all nodes. My change is below.

function my_form_configy_node_view(array &$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {

  $config = \Drupal::config('my_form_configy.settings')->get('checkbox');
  // add the cache tag, so that the output gets invalidated when the config is saved
  \Drupal::service('renderer')->addCacheableDependency($build, $config);

I am trying to understand what this does? Where is the value of 'checkbox' being stored now. Its not in my modules settings.yml file because I have not exported the configuration yet. Is it stored in browser cache, or Drupal cache? Since I am developing I disabled all twig and drupal cache in settings.local.php with.

$settings['cache']['bins']['render'] = 'cache.backend.null';
$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';

The Devel module is not using addCacheableDependency

Clicking the checkbox 'Display $page array' The dpm feature works immediately.

function devel_page_attachments_alter(&$page) {
  if (\Drupal::currentUser()->hasPermission('access devel information') && \Drupal::config('devel.settings')->get('page_alter')) {
    dpm($page, 'page');
  }
}