How can I find app/AppKernel.php in Symfony 4?

I want to register a bundle in the AppKernel Class in Symfony 4 according to this tutorial:

https://symfony.com/doc/3.3/bundles.html

But I do not find a folder "app", and also not the file AppKernel.php, so not the class AppKernel. Did I made a mistake by the Symfony installation or do I need to create an app folder by myself?