What does "to be on it" mean?
There were two girls, their car had been stolen/ One of the girls recalling the situation:
“Catey, you were so on it,” said Sophie. “You just stood there and you looked around and you saw this convertible. You pointed at it and you said, ‘The Lord provides.’ ”

What does "you were so on it" mean? Like the girl sorted out their problem?

Question

What does "to be on it" mean?
There were two girls, their car had been stolen/ One of the girls recalling the situation:
“Catey, you were so on it,” said Sophie. “You just stood there and you looked around and you saw this convertible. You pointed at it and you said, ‘The Lord provides.’ ”

What does "you were so on it" mean? Like the girl sorted out their problem?

Answer

To say you are"on it" means you will take care of a situation. It means you are doing something that needs to be done, or you are trying to solve a problem.
"You need to get your homework done." "Don't worry;I'm on it."

Answer

I think the others have got the meaning, but, personally, I would never use "I'm on it" that way. I'd say it only in the present and without the "so".
She's on it. (she's taking care of it)

Maybe the young use it that way. It has a hint of admiration. (I'm impressed with your capabilities)

Answer

I suppose "be on it" means "be in charge of something". So, "you were so on it" means "you were the one to take care of the car".

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!