To grow Popeye forearms, you will need to pay attention to a lot of different muscles!
Your forearms contribute to all wrist movements and to elbow flexion with the help of the biceps.
If you want complete development, you need to work your wrist flexors, your wrist extensors, your finger flexors, your finger extensions, ulnar deviation, radial deviation, pronation, supination and elbow flexion.
As you can see, there are a lot of muscles in this region of the body and working them all individually would take a lot of time!
Consequently, some compromises have to be made in order to get big forearms without spending too much time isolating each muscle.
The first exercise that I recommend is the pronated grip curl, you can do it with a barbell or with dumbbells.
This movement will mainly work your brachioradialis which is an elbow flexor and the biggest muscle in your forearms, it will also engage the wrist extensors.
It also has to be noted that when you hold onto a weight, you work your finger flexors and this is the reason why I do not recommend doing specific work for the grip.
The second exercise that I recommend is the supinated grip wrist curl which will help you develop your wrist flexors.
If you do those two movements, you will get the biggest bang for your buck possible!
But to be as complete as possible, you could even add another exercise which is the wrist deviation.
You simply stand up with dumbbells by your sides and do a radial deviation followed by an ulnar deviation working those two functions simultaneously.
For all of those exercises, I recommend doing moderate to high reps due to the fact that most people have a poor mind-muscle connexion with their forearms.