A custom build of Android for an existing device. How difficult?

Question

Android

Hey!
Couldn't find an answer to the question anywhere. There are many Chinese (and not so) devices, the Android assembly on which is usually very raw and crooked. Why does no one make their own assembly for them, but at best they try to finish the existing one?
It is clear that we need drivers for the hardware. Is it really impossible to get them out of the factory firmware? Is it really possible to create on your own a normal assembly of Android for a specific device, with normal power saving, etc., without having experience in Android?
PS The question arose because all these mods of the original firmware usually look like a collective farm (partly because the changes are often provided by additional software, which, of course, is poorly integrated with the system itself) and they do not radically fix the firmware - rather, they add all sorts of far from always necessary bells and whistles.
PPS I myself am a C++ programmer, mainly for Linux. Therefore, I will only be happy with the technical details.
PPPS There is a Chinese car radio (MTCD) on Px5. I want to experiment with her.
Thanks in advance for your replies