code: public function onKernelTerminate(PostResponseEvent $event): void
      {
          if (!$this->framework->isInitialized()) {
              return;
          }
  
          $request = $event->getRequest();
  
          // Only index GET requests (see #1194)
          if (!$request->isMethod(Request::METHOD_GET)) {
              return;
          }
  
          // Do not index fragments
          if (preg_match('~(?:^|/)'.preg_quote($this->fragmentPath, '~').'/~', $request->getPathInfo())) {
              return;
          }
  
          /** @var Frontend $frontend */
          $frontend = $this->framework->getAdapter(Frontend::class);
          $frontend->indexPageIfApplicable($event->getResponse());
      }
docs: Checks if the request can be indexed and forwards it accordingly.