z-index be like

The reason the force is unstoppable is because it doesn't interact with anything. Otherwise, interaction would reduce the force.

I always assumed the immobile object was made of an impenetrable substance. Force is also always fundamentally an interaction between particles.

That's not to say there is no change to the system. What builds up is energy. Depending on the two particles in the system and the vectors involved, quantum tunneling is possible and the particle exerting the force passes into the object. Then the system changes.

The unstoppable force is simply going around the immovable object on the other side that we can't see in a 2D view.

It's like an unstoppable force meeting an immovable object. And getting brunch. And really hitting it off.