I don't know what to tell you here, Bob. You're pretty close.
Your going in position is that you're not convinced of major differences between the distributions, yet it sounds like you haven't tried many.
GNU/Linux (the canonically correct name) is an operating system consisting of the Linux kernel, GNU and other FOSS replacements for unix building blocks and various utilities and a desktop environment.
Various distributions primarily change those last two items in the list above.
The desktop is actually optional on any *nix operating system. A useful and desirable option but an option nonetheless. Last Ubuntu I ran, I replaced the desktop with one more to my liking.
It's a completely different paradigm than various Windows versions and you need to accept that if you want to understand it.
You can sample distributions until you find one you like, or you modify any of them to the limits of your experience and develop whatever user environment meets your needs.
You can even take a Linux distribution optimized for embedded applications, add a stack engine to serve as a virtual machine, supply utilities that run in that virtual machine, and call it Android. Google did.