How could you do a clean system reset after someone had access to all installed software/data including the cryptographic keys? The information is gone, maybe the recovery partition is changed. How could you securely recover?
I'm freelancer and helped to develop some head units. I have a surprize for you: This documentation mostly doesn't exsists. Most of the time there are some chip datasheets and requirement documents, depending on the customer(car manufacturer) they are good or bad and then are some partly outdated wiki pages written down for some important special things. You learn all other stuff out of the code or from your colleagues.
Wait two years and the most knowledge is gone, except of the things that are used for the next head unit.
The biggest advantage actual developers have is access to the NDA'd vendor docs and the official SDKs. And, the vendor docs are bad and the official SDKs are a mess. Internal documentation? You'd be lucky if it's two steps above "nonexistent". It's usually just one step.