They (/we, I'm just testing armour modeling now) use a body model, otherwise we wouldn't know any measurements/body proportions, or at least that's how I think everyone is doing, don't know how the heck you would make them otherwise
But all armours are made from scratch if that is what you meant, of course, if you have made an armor you could technically use it to help you make a new armour, but it's often easier to remake from scratch.
Any program is good, use the one you are most used to, I use 3ds max, some use blender i guess (I dont like blender >_>) etc etc.
//edit
I downloaded the environment armour and I'm not sure how the skin works :S A part of the armor was modeled (like normal) but a large part of the "armour" was just a texture on the skin. Was it an in-complete version or what :/