"It's going to go inside something that we use in CAT scans and MRIs of whatever is their good arm, to make a silicon rubber, then coat it, and paint it in 3D, exact mirror image of their other limb."
The first "It" may be a prosthetic arm. But I don't have an idea about "go inside something that we use in CAT scans......." How does this mean?
This is almost impossible to understand.
Your comprehension is not the problem -- it's the bad writing.
I don't know what it means either, with all those its.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.