Hey Aditya !
This is Yogesh Malhotra. I have been pursuing my articleship from last 4 months from Deloitte Haskins and Sells-Delhi Office.
In the beginning stage of your training you are made to work under your seniors to learn and gain experience. The work is delegated to you on the basis of your capabilities and knowledge.
And after certain specified period of time, which may vary from firm to firm, you are made to handle one complete audit or audit area.After you complete your work, it is then reviewed by your seniors for further improvements.
That is how the work is done not only in big4 but in any of the C.A. firms.
Please mind that I have told you only broad parameters.