Not OP but are you saying the google's "Android" OS, which comes installed in smartphones that aren't called iphone, is open source? because it is not. There is an open source version of it which is not the one you have access when you buy a phone.
What runs in your phone is AOSP, with some firmware blobs (many of which are proprietary), with some system apps that may be proprietary (like Play Services and Play Store or whatever the OEM adds). If you need to convince yourself, try to build AOSP and run it.
The OP is just trying to derail the discussion. Would you say that Linux is not open source because the Linux you run on your computer most likely has to run some proprietary blobs? Or that Linux is not an OS because Linux is a kernel? What would it bring to the question of "are there open source OSes out there?", really?
I'd say SteamOS is not open source IF SteamOS was not open source (but it is).
The Android that google ships to phones isn't open source the same way the Google Chrome browser isn't open source either. To the license level. This is quite obvious, not sure if you don't understand or simple don't agree (and prefer to be wrong).
Do you even know where the discussion comes from? After enumerating "macOS, Windows, Linux, iOS, Android", it was said:
> it would be best if developers all over the world boycotted proprietary platforms entirely.
I noted that Linux and Android are open source.
To which I got the answer:
> Android is closed-source but with some open-source components, for only some of which they'll actually follow the license and give you the source code.
Now please explain me how "they only follow the licence and give you the source code for some open-source components of Android" is obvious, and how I choose to be wrong when I say that the full goddamn OS is open source, not because Google follows the licence (because they don't have to, they own the codebase) but because Google chooses to publish it. There are many reasons for that and I hate Google as much as the next person, but the fact remains that the Android OS is open source.
So back to the original point where "developers should boycott proprietary platforms", I don't think that Linux and Android should be boycotted. Developers should use and support open source Android systems like GrapheneOS and LineageOS (which they generally have no idea about) instead of boycotting them.
I dare you to buy a random Android phone and try to get the source code for the GPU driver that will compile and match the functionality of what's installed on the device.
Android is a good proprietary platform so locked that in the near future it'll be impossible to "sideload" apps (install apps without Google first allow it). Google Android isn't open source.