And "Windows variants"? Comparing Windows backwards compatibility to the ability to run Android apps across different Android forks misses the point. All versions of Windows are proprietary operating systems owned by the same company: Microsoft. Amazon Fire OS, Lineage OS, and other Android forks are not.
Microsoft does license Windows to the same OEMs that are having fun with Android, which just like on Android are allowed to do a certain level of customisations.
Windows is not only what comes with a PC.
The only big difference is that AOSP costs zero dollars, euro, yen,.....
The difference is that it's free and open source. ROMs such as Lineage OS do not need to pay for a commercial license to fork the code, and an app such as F-Droid can run natively on any Android device.