Android is open source. Nothing stops anyone from making changes to the os for their phones including removal of the play store. Google just prevents them from using the android trademark if they don’t fall in line with their comparability standards. Technically speaking they could hit every metric of compatibility with the exception of the play store and third party apps would run on their os exactly the same. All it would take would be a smart marketing campaign to explain it. So they don’t have to have it on it.