Set the page title programmatically in Drupal 9

Change on a form or controller

Just set ['#title'] element.

Form Example : $form['#title'] = "The title";

Controller example:

public function page() {

    $output = [];

    // Set title.

    $output['#title'] = "My new title";

    //

    // Your codes ...

    //

    return $output;

}

Change using route and request

Example:

          $title = "The new title";

          $request = \Drupal::request();

          if ($route = $request->attributes->get(\Symfony\Cmf\Component\Routing\RouteObjectInterface::ROUTE_OBJECT)) {

            $route->setDefault('_title', $title);

          }

Get the page title programmatically in Drupal 8/9

$request = \Drupal::request();

if ($route = $request->attributes->get(\Symfony\Cmf\Component\Routing\RouteObjectInterface::ROUTE_OBJECT)) {

  $title = \Drupal::service('title_resolver')->getTitle($request, $route);

}