code: public function loadFromPlugins(): RouteCollection
      {
          $collection = array_reduce(
              $this->pluginLoader->getInstancesOf(PluginLoader::ROUTING_PLUGINS, true),
              function (RouteCollection $collection, RoutingPluginInterface $plugin): RouteCollection {
                  $routes = $plugin->getRouteCollection($this->loader->getResolver(), $this->kernel);
  
                  if ($routes instanceof RouteCollection) {
                      $collection->addCollection($routes);
                  }
  
                  return $collection;
              },
              new RouteCollection()
          );
  
          // Load the app/config/routing.yml file if it exists
          if (file_exists($configFile = $this->rootDir.'/app/config/routing.yml')) {
              $routes = $this->loader->getResolver()->resolve($configFile)->load($configFile);
  
              if ($routes instanceof RouteCollection) {
                  $collection->addCollection($routes);
              }
          }
  
          // Make sure the Contao frontend routes are always loaded last
          foreach (['contao_frontend', 'contao_index', 'contao_root', 'contao_catch_all'] as $name) {
              if ($route = $collection->get($name)) {
                  $collection->add($name, $route);
              }
          }
  
          return $collection;
      }
docs: Returns a route collection build from all plugin routes.