"Is X-Plane 10 Digital Download on x-plane.com same with X-Plane 10 Global - 64 Bit on Steam (2014)"
"I found plenty of DLC on Steam for X-Plane 10 Global - 64 Bit; are they related with Global Scenery Upgrade for X-Plane 10 Regional Editions on the x-plane.com?"
"is X-Plane 10 Digital Download on x-plane.com global or regional?"
It is global.
With X-plane 10 Digital Download purchased on x-plane.com, you will receive a product key that you will be able to use with the installer you can download here :
I would recommend you try the X-plane 10 demo first, to see how X-plane 10 will perform on your computer. Make sure you have more than 20-25 fps :
There is a demo for X-plane 11 too.
Be aware that there will be no future update for X-plane 10.
In any case, don't be too disappointed you can't buy the Steam version :
-You are directly supporting the developers of Laminar Research if you buy the Digital Download from X-plane.com
-bugs are easier to understand if you don't have Steam in between.
-you will understand better the structure of X-plane, and it is always an asset, to understand why this or that doesn't work.
-you will have the same choice of additional third party products. See X-plane.org for that. Moreover, if you buy a scenery for X-plane 10 and you decide one day to upgrade to X-plane 11, you will still be able to use your purchase. This wouldn't be the case through Steam.
-this is not the case for X-plane 10 anymore, but you receive updates faster (final or even beta updates if you wish) when owning the Laminar Research edition.
Welcome to X-plane and all the best to you.
The X-plane 10 online manual is here :