There is not a multitude of phone manufacturers using an open source operating system.
The Android that phone manufacturers use is largely closed source, and only part of it is open source.
This has been irrefutably documented elsewhere in this thread.
This thread is about phones.
The fact that you can use the open source components of Android to build a home security system, but not a phone, just proves the point, that Android as a phone operating system, as most people consider it to be, is not open source.
You only have to look elsewhere in this thread to see all of the features which have been moved out of AOSP into Google Play Services to see this proved.
Whether it boots or not is irrelevant if it doesn’t do what people expect a smartphone OS to do.