Squats (may) help in eliciting hormonal responses and definitely increase appetite, which is necessary to put on size.
To actually put on arm muscle mass though, I always had the best results on my arms from going to failure/doing drop sets/a complex on tricep/bicep isolation movements ~twice a week for each just at the end of a workout. I was able to do this while doing push/pull programs or powerlifting style programs. If I was doing a program where I push and pull on the same day, I would superset bicep/tricep isolation at the end. Typically those programs would have me doing it 3x a week. I was never going to failure on the compound movements that comprised the rest of my workouts, so my body always felt fresh; the triceps/biceps might be sore the next day, but rarely ever two days after. This allowed me to hit them hard and often without comprising my main lifts, which also stimulate the arms at varying rep ranges/intensities to a certain degree.
Having a sole, dedicated arm day has also worked, but it never really fit in with my goals as I found it difficult to program around my strength goals.
Caveat: I have terribly unimpressive arms at the moment, just gonna have to take my word for it. Or I could show you a reverse before and after where you can see how much I lost by not doing what I outlined hah