Yes, that root is called NeXTSTEP, aka. everything Apple since they bought that and build everything they have on top of it.
Oh so very absolutely not. That root is called Common User Access. When Motif was deciding precisely how they were going to implement it, Microsoft was a member of the Motif Working Group. Motif (which underpinned every Unix desktop newer than Sun OpenLook) and Windows grew together, which is why their behavior is nearly 100% identical and their looks are also immensely similar. fvwm, which dominated early Linux GUI configurations (the majority were based on it) was by default both visually and functionally a clone of mwm.
Ironically, NeXTStep also implemented most of the CUA, and Mac OS X was a huge step away from that and also a huge step backwards in overall usability. The biggest fail in OS X was the way they ruined the Dock. At a time when computers were transitioning from 4:3 to 16:9 screens, and vertical space was at a premium, Apple moved the Dock from being vertical on the right hand to being horizontal on the bottom. This was exactly the wrong time to do it, and they did it in exactly the wrong way as well: By redesigning the dock for eye candy by making everything move around all the time, which destroys your ability to use muscle memory to click on elements you need to access on a recurring basis. Now you have to slow down to look at things which are changing in both size and position.
In the bargain, Apple has also somehow made the GUI slow to respond to input, so it feels like swimming through treacle even on very fast hardware. NeXTStep was fairly responsive on a 68040, but I find myself constantly wondering what the holdup is with Mac OS in a way that I haven't since I used... classic Mac OS, where you'd commonly just have to wonder what was taking so long since there was typically no indication what was choking your entire system even when you were using the multifinder.
Pity they didn't choose BeOS, instead they got the juice fasting turtleneck and ruined NeXTStep, which was so beloved in Nerddom that it spawned countless visual imitators and copies including Windows 95. I used a NeXT themed WM and nextaw3d on my 386 running Slackware...