Except OEMS don't ship Windows forks with their own SDKs, UI frameworks, custom L&F, alternative stores.
Contrary to Google, Microsoft has a clear position on what OEMs are allowed to do with Windows.
Google lets these forks do anything they want except things that take away money from Google.
Android OEMs largely don't do that either except for some specific markets where Google isn't a thing. Google does have rules for when you want to be certified and have Google Play and Google services on board, which you mostly do in the West and so OEMs comply.
Sure, they may ship a Samsung skin or preload some of their own junk, same as on Windows. That doesn't make what they ship a different OS.
The one exception would be HarmonyOS, but that's somewhat China specific.