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.
AOSP means "Android Open Source Project". You can get the sources of Android, modify them, and build the whole OS. The Android SDK is open source as well.
GrapheneOS is based on Android precisely because Android is open source. They couldn't do that with iOS, or Windows, or any proprietary OS. Same for LineageOS.
Honestly it seems like you don't really know what you are talking about here?
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).
GrapheneOS is based on Android precisely because Android is open source. They couldn't do that with iOS, or Windows, or any proprietary OS. Same for LineageOS.
Honestly it seems like you don't really know what you are talking about here?