I used plenty of references from Pinterest, including one that gave me the whole design for the knee and lower leg. I'm sure you'll recognize that the idea is pretty much lifted from here. The thing is that I made it my own by re-drawing everything differently to my tastes (and from the back!) and rendering it completely differently. A successful piece typically amounts to such a wide array of "theft" that it stops being thievery - it's a remix. Steal like an artist, my friend!
As for learning how to do mechanical designs, there are two things. I'm a mechanical engineer (oops lol!) by trade so I have a trained eye for what "looks right" and what does not. You can skip the engineering school step and surpass me entirely if you simply analyze and consume the right kind of art. Mecha designs, space ship interiors, even existing products.
I'll give you my two cents here - most of the stuff that passes for "futuristic" in sci-fi machinery design looks obsolete in terms of mechanical design. At the very least, immersion breaking. We should be using bike helmets as reference, not steam engines. Think 3D printed titanium lattices, not screws and bolts. The shitty old mining equipment I've maintained on remote sites in Wyoming looks more elaborate and complicated (and appealing!) than a lot of "futuristic mecha" paintings on Pinterest. Go figure!
On another note, how to make a good design. Create patterns in the machinery - a repeating triangle shape, or a re-occuring slotted hole that's all over the place. Perhaps it's 45 degree angles in all the panel line corners? Bevels? That's all part of shape language, and it will help bring mechanical things together in a sense that they look like they were designed by the same team and most importantly manufactured by the same entity.
More of my rants on mecha design can be read here, in this previous post.