Most PHP frameworks are installed as Composer packages. They end up in vendor/, behind a version constraint, and you interact with them through their public API. You don't read the implementation. You don't modify it. When something breaks after an update, you adapt your code to match the new version.
It happens often with frameworks: you follow the code, everything is fine, then suddenly... vendor/. No more visibility, you have to guess what happens inside, and it can be frustrating when you are learning.
Not the documentation. The code. The one that actually runs in your project.
If you use a "classic" framework, that code is hidden somewhere in vendor/. Abstract, distant, untouchable. When a new version ships with a breaking change, you deal with it. When a behavior does not suit you, you work around it. When you want to understand what is really going on, good luck.