doesn't often work like that.
A lot of thingiverse designs are via openscad. And some people will post the .scad files alongside the .stl file. I don't :-)
And some people will post the files for whatever other cad package they use.
Desingning a phone case from scratch in openscad is pretty easy. But then that's pretty much the only cad package i use - so i would say that. :-)