Is Android any different from Windows in that sense?

It's open source in name only. All development is done behind closed doors and essentially dumped out in the open when Google determines it's ready for a release. No one else has say in the development process and you can't contribute changes either.

All the other things you mentioned, can also be done on Windows. I am not sure how that's different than what Windows does/is doing.

1. Most of Android is Open Source - see Amazon Fire, for example. Is there an equivalent of that on windows?

2. MS took apple's approach that everything (on their new platform) must go through the store.

Even if right now they give exceptions, it's as I said above an example of them trying to build market share. Will they have an "Unknown Sources" button when they get 80% of the market?

Openness is about being able to modify it and share it with people, not about being able to get Android to adopt it.

For a long time even the GNU project had a similar cathedral-style approach of secretive development processes and no outside contributions.

You might have a point when there is a CyanogenMod/LineageOS equivalent for Windows.

And then there's Chrome...

What are your objections to Chrome? Chromium seems like a reasonable project and everything that's not in Chromium seems to be pretty well Google specific and probably shouldn't be in Chromium IMHO.

which is 99.99% open source (Chromium), and forms the basis of several competitors to Chrome?

compared to the openness of Edge/IE, Google are saints with Chrome

> (Chromium), and forms the basis of several competitors to Chrome

Chromium was based on Webkit which actually came from Apple (and who forked it from KHTML from the KDE project). It's not Chromium that's the "basis" of competitors, it's Webkit that was the base.

By that logic, KHTML/Konqueror should get all the credit. But AFAIK V8 (which most of those forks use) also has pretty much nothing to do with JSCore.