Drop all isolation lifts for arms??
-^There is exactly what not to do. Captain's arms grew, if i can remember 2 1/2 inches from doing this with none to little isolation work. 2.5 inches is a lot, but this is from someone who trained for 6 years on and off non-consistently. He claims this growth came from the heavy compounds, which i agree it did to an extent, but remember, this is from a non-consistent lifter. Most of those gains on his arms were due to him starting to lift again.
Your question in this post clearly states that you are a beginner lifter, so therefore your arms aren't going to be super thick already. Yes, you might have an impressive peak, but that may just be due to your low bodyfat and lack of food consumption. How often do you see a bulked up bodybuilder with over 15% bdft with a huge peak? usually never. It's not noticeable until they cut down. ME on the other hand, feel like i have thick arms, and not enough peak. (we can trade some thickness for peak if you want..haha)
So if you are worried about the thickness of your arms/sides of your arms, your grip will have a lot to do with where you are primarily hitting the bicep. Bicep is 2 muscles, inner and outer. Maybe next workout start with a close grip or wide grip and don't worry so much about a neutral grip for hammers of a shoulder width grip until the end of your workout.
On the other hand, i do agree with Captain. A lot, if not most of your arm development will come from compound lifts. But, i do not agree with completely eliminating every isolation movement for your bicep. After your back workout try getting in 2-3 exercises for biceps with around 3 sets for each exercise. Remember, you are 19 and still young(im young too), and bicep thickness/arms thickenss will take some time to develop, especially if training naturally.
*Focus on training as much as nutrition and calorie intake and you'll start seeing some bicep thickness soon! Good luck!