Google own the merge rigths, and what goes into each version. and their process timings ensure that by the time you patch is out, your device is not getting updates anymore (update timeframe that they set both in contract with oem or via "example" via their nexus series)

What merge rights? You can build Android yourself and install it directly on your phone, and let others do the same - unless if the OEM's and ISP's gates prevent you.

unless "your device" is the emulator, there is no way to get hold of drivers or firmware for anything other than lifting them of an blessed rom ... and only if you can get the parity you need with the kernel you are using.

there's a very good reason the "supported device list" of all forks is always very, very small (even with the outlandish claims on most of them such as "supported. touch screen and modem still not working" )

just becuase the build steps for your fork project automates some of that driver/kernel lifting, doesn't make it any better.

And all of that is also gatekeeping by the OEMs.