Android is "fully open source" except that Google writes 99.999% of the code in secret. Rarely they will accept a pull request but there is zero transparency into that process.
Which parts are they coding in secret? (Honestly, I don't know, please help me understand)
[0]: https://android-review.googlesource.com/#/q/status:open
When i buy a Laptop from HP, Dell, Lenovo or any other OEM, i still get Windows updates (even if i don't upgrade to the latest Windows version). I would really like to know why it is not possible for Google to implement such an update system? Blaming carriers is easier i guess.