Symfony how to translate files from yaml to php?

Question

symfony

Hello. I sat down to learn Symfony 4. I put it on my local server. But I got stuck on the first step. https://symfony.com/doc/4.0/page_creation.html . My routes file is not in php but in yaml. Why is it in this format and how can I make sure that all working files that can be used are in php? (From the frameworks, I used to only tinker with Lara a bit, everything was in php there). I feel like this is a noob question, but still.