The section Creating Routes in YAML, XML or PHP Files should to be updated for Symfony 5.1.

The change symfony/recipes#705 removed the support to mutiple file extensions routing configuration, taking only on .yaml files by default. So, if you want to configure your routes using a routes.php or routes.xml, you will need to adapt the method configureRoutes on Kernel.php to work with the file extension you want.

The documentation do not mention that change, misleading the readers to think that, as in previous versions, they just need to create a config/routes.php file to make the routing work.