Armour patterns are pretty much the same theory as fabric patterns. Think of a top with princess seams - you're using a flat pattern to make a 3D object. In armor you just have the use of a bit more flexibility in your material and it being able to retain shape in a curve instead of buckling to an extent.
The only armor piece I really see in the reference is the arm thing. The calf-protectors look like pieces of perforated leather. For the arm thing you're really looking at a lot of rectangular pieces to wrap around the arm, held together by joints or something else with circular pieces around them for decoration. I can't see what's going on at the shoulder well, but it looks like some rectangles with pointed ends, basically. Not sure if the elbow is functional or what, either - I'd want to find more reference pictures if I was making a pattern. A good shot of it from the side would help.
It's really just like sewing and making sewing patterns. Think, "what do I have to do to make this 3D" ? Think about what shape it will have to be when it's flat. You start with making your pattern out of cardboard or even construction paper - just like you'd make a toile of a fabric costume out of muslin.
Unfortunately when it comes to armor that someone just drew from imagination it's often times impractical/nonsensical - so you have the challenge of making it work best in real life.
And instead of sewing seams you're usually working with joints, glue, or elastic - but they're not any harder than seams, and in fact I think they're easier !
I would not classify the bow as armor because prop-making, while you can apply pattern-making to it to an extent is pretty different. You're usually building up from a base or you're doing something entirely different like creating a form and casting and molding (which can be done for armor, too but is less common - more for things like mechs).