Hi

아저씨 is calling the old man
저기요 means  calling someone (it could be a woman, man, student but they are people who are not acquainted with you)
I'm glad you're studying hard Ava ~! :)

저기요 can be used to mean "excuse me"(or even "hey" ) to draw someone's attention.
for instance, at a restautant you could say " Excuse me, I'm ready to order"  (저기요, 주문할께요 )
or in a street "Hey(excuse me) , you dropped this!" (저기요, 이거 떨어뜨렸어요!)
Hope you figure it out well!