It's interesting how while Microsoft is moving to consolodate Windows 10 across desktop, tablet, smartphone, Xbox, etc, Apple is ever expanding int more OS's. iOS, OS X, Watch OS, and now tvOS?? Sounds very cumbersome... but perhaps that's better than lumping them all into one all-encompassing OS?
It is weird. Behind the scenes (and to a lesser extent, in your face) Apple keeps merging iOS and OS X. As new APIs come out, they're identical between the two platforms. Old APIs for iOS and OS X have each steadily been getting rewritten to mirror more current ones for the other.
I haven't touched watchOS... how similar is it to iOS?
The main two are:
OSX - Intel based Chipsets
iOS - Arm based Chipsets
Watch OS - Arm Chipset Extension - not a separate OS
TV OS - likely and iOS Arm Chipset Extension - But possibly listed as a separate OS in Xcode to compile to.
Then you have all the Kits - HomeKit, ResearchKit, HealthKit, iBeacon that work across all OS's
Microsoft haven't got 1 os. They have just named it all windows 10. They infact have loads.
Windows 10 Windows 10 Professional
Windows 10 Enterprise
Windows 10 Education
Windows 10 Home
Windows 10 Mobile
Windows 10 Mobile Enterprise
Windows 10 for phone
Windows 10 IoT Core
All of these separated by a break are separate OS's in order of features but obviously share a lot. But you won't be able to run a win 10.exe on Phone or IoT. They will need a separate compile and Win 10 proper has a lot more features that can't be used on the lower foot print ones.