In one thread people fighting the ever decreasing amount of hw ownership of most devices in our lives and when we have one that is more open, the crowds come to attack that too. The theat model with tech has always been that if an attacker has physical access to the device and time then it's game over.
Because it's not open for modification by the general public? (emphasis general, not just technically minded people) Manufacturers need to pick a lane - either fully open, and then people who need it can harden their own stuff (and at least be aware of the tradeoff), or fully closed and secure. This in-between where cars are invasive privacy nightmares that spy on you at all driving hours, and are insecure nightmares…
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and requirement documents, depending on the customer(car manufacturer) they are good or bad and then are some partly outdated wiki pages written down for some important special things.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.