pull ups

what is the debate conclusion that we've come up with over the past couple decades when it comes to isolating the arms. Have people come to the realization that isolating them is more or less efficient than just doing compound movements at work the body part. For example is doing bicep curls...

I'm 22 years old and I've always been super weak on pullups, i can remember in PE they laughed at me in high school cause i could barely do 1. now I can do like 5. I see some older dudes that can do like 40 in a row like nothing. what is an easy routine that i can follow to improve on these?