I have gathered them in the \Magento\Framework\App\FrontController::dispatch():
\Magento\Framework\App\FrontController::dispatch()
Magento\Framework\App\Router\Base
Magento\Cms\Controller\Router
Magento\Framework\App\Router\DefaultRouter
The base router (Magento\Framework\App\Router\Base) ordering has been changed from 20 to 30 in Magento 2.2.2 at 2017-10-15.