I dont specifically do forearm exercise
But they got hit very very hard during deadlifts, rows(bent over, seated cable, dumbbell), weighted pullups and couple of other exercises
Mind you wrist wont grow because well they are bone
One rule when you are doing pulling exercise, grip as hard as you can until knuckle turns white, not only it prevents barbell from rolling out , it also trains your grip and forearm, at the same time helps you exert force to it
Use double overhand as much as you can, until you fail, apply chalk to it and continue double overhand, fails again only switch to mixed grip, fail once more again only seek help from straps