Heavy ass deadlifts and shrugs. Additionally chins will wreck your forearms. Done properly without pussy-ass straps you won't need any auxiliary forearm work.
Equally amazing is the fact that this is the first time I've seen a post by you where you are not tooting your own horn on how many magnificent new shoulder exercises you have invented.
The act of a muscle repairing the damage you do to it during resistance training. A muscle over-reacts so to speak, and repairs the damage done, then adds more muscle on top of it.