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.
Harmony NEXT is no longer based on Android.
If I buy a Samsung phone in North America or Europe today I get Google Play as the main app store, Google services etc. and if I want to develop an app for a Samsung Android phone, I use the official Google SDK and Kotlin like for every other Android phone.
They have their own skin and store, but nobody really uses that. Google Play it is, be it Samsung or Xioami. They're very far from offering any kind of genuine alternative to Google Play, Google or Android. Their stores are junkware no different than ASUS preinstalling some antivirus nobody wants.
> Harmony NEXT is no longer based on Android.
That's my point. Huawei is the only OEM offering something different, but not easy to get in Western markets and thus doesn't have app mindshare.
Google SDK naturally doesn't have the support for OEMs specific features.
Leaving just one Samsung example here, too lazy to dump everything else,
https://developer.samsung.com/mobile/galaxy-sdk-getting-star...
I am yet to see anyone disable OneUI, MIUI, HyperOS, EMUI for bare bones AOSP UI.