An important thing to note right from the start is that Contao can be used as a standalone Symfony Bundle in your existing Symfony application. So if you need to expand your Symfony web application with the functionalities of a Content Management System, head over to the Symfony Application article to learn more about that.
Otherwise, when installing Contao from scratch, e.g. via
composer create-project contao/managed-edition
or via the Contao Manager, it will still be a Symfony application – the Contao Managed Edition. Within that setup you are still able to use the Symfony Framework and its Bundles to their full extent, while also having the possibility to auto-load and configure bundles for example (e.g. for Contao Extensions). Head over to the linked article to learn more about the Managed Edition.