Best self-described mental model for how my brains feels internally is as objects (in the programming sense) with properties and relations. As I "imagine" something, I'm recalling the object "living room couch", and then enumerating properties of that object "leather, brown, L shape."
If I think of something on the couch e.g. "cushion", then it starts off being just a relationship "cushion is on the couch" without any location component. If I then inspect the "location" property of the cushion, then I think "the cushion sitting upright in the L-shaped bend of the couch."
I'm pretty damn good at spatial reasoning in my head, e.g. I can "design" a component in my head, and then sit with Fusion 360 and create it as a 3D CAD model... but I'm not actually seeing anything in my head at all. It still remains in that abstract object <> properties <> relations head-space.
In some ways, when I'm imagining a 3D object in my head, I'm actually thinking "this sketch" with "this shape" extruded by 10mm. So the actual constructive process by which an object is realized in my mind IS the actual process by which I'd design it.