How to replace prefabs with blender models?

Question

Unity

I’m currently making a kind of engine, that is, paths are planes, trees are cubes with spheres, opponents of a cube, well, I think first to work out all the interaction and movements and then replace these prefabs (cubes and spheres) with models ...
The fact is that with I didn’t work with a blender and the like at all and I don’t know what and how it is, so the question is whether it will be possible to replace prefabs with models later or ... or how is it right to do all this?
And now I’m writing this engine, and then it turns out that I need it differently ....

The engine is figurative, that is, I write the frame of the game, that everything worked there, walked, shot, recorded, calculated, etc.
But there are no models themselves, that is, for the place of cube buildings, for the place of units of the sphere, etc.
And then I thought about adjusting textures and models...