@ljrk @joshbressers @kurtseifried The core Android portion of Google is very much NOT out-of-tree, they merge their code upstream almost always first before applying it to older Android kernel branches. See the yearly report at the Linux Plumbers conference for the past 5+ years where they what has been merged, what is left to be merged, and what is coming next, with pretty graphs and the like.
They have a strong "Get your changes upstream first" rule with their OEMs and you can see the interactions with those companies uptream has increased dramatically over the past years been because of this.

ChromeOS portion of Google also has a strong "upstream first" rule, and is very good about taking updates and getting changes merged properly.

The Pixel out-of-tree stuff is due to other "reasons", but that is a separate division and there have been many patch submissions recently upstream to resolve that delta, so that is being whittled down.