I do not know how the professionals do it. I am relatively new myself to the 3D stuff. But here is how I do it.
I design each piece of my model separately ( Wheels, Fenders and etc. ) then I assemble them into 1 model. I do not weld any of the meshes together. I attach a node to each section, then I animate the model simulate explosions or damage. By not welding the meshes together, I am able to pull them apart from each other in the animation sequence.