Those are actual real 3d renders (done in MODO Indie) with slight postprocessing in GIMP.
Basically, everything is 3d modeled.
Here is a wireframe of the port:
After you model it, apply materials, do some simple lighting, render and voila ... something pops out

I always render some other layers except colors and shading (like ambient occlusion, material ID, incidence angles etc...). I use these additional layers in GIMP for giving the images some more interesting looks. Do some color correction to get the tone and atmosphere I am going for and that is basically the whole process.
It is not hard it is just time consuming and I do not have much time to spare.
But, one thing is sure, I do not plan on giving up the GFX work for Freeciv, I am doing it now for so long (since 2014) that it has become a life style
