If I could only figure out how they engineer the ball joints, etc. for connecting the pieces.
You design the angle constraints for the joint according to the attached part or base and the connected member needs, and then you propose dimensions for them. You need to know the material's strength (baked modeling clay/plastic/white metal for this cases) as well as their friction coefficient to calculate the forces applied to the joints (how fast it will receive loads, inertia, etc -basic physics,-) and determine -according to your proposed dimension for the joints- if the base joint will withstand the forces applied to it. For this, you will use vector calculations for force systems.
I'm an architect and that's how you would design those types of structures, regardless if it's a building structure or a toy joint. But one is probably better off using the trial and error method for this application.
This is interesting since it reminds a little about scale modeling for interior details. You can use white metal casting to achieve impressive details for robots. Good luck to the guy designing Prime!