Almost all of Chrome is open source. You're confusing it with Safari, where the engine is open source but the browser UI isn't.

It depends what we're after. Almost all of the code in LoC percent terms might be open-source, which helps people who want to launch derivative browsers or learn from the code. But on the other hand, almost all of the Chrome instances running today are proprietary builds with mystery special sauce added (I'm guessing desktop Linux users are the only significant userbase to run vanilla Chromium), which doesn't help us on privacy, neutrality, etc...

Almost. There is no confusion, rest assured. They are both proprietary, but one of them likes to act as if it was open source (see also: Android), and clearly it hides well.

Android as far as I know uses a source-drop model. For both WebKit and Chromium, all code reviews are as far as I know done publicly.

AOSP is open source, but Android itself is far from it - just ask Huawei. More and more code is in proprietary bits of Google code. And Chromium is not Chrome, most people use Chrome builds which are proprietary.