most of the IITs did not conduct interviews. They offered direct admission via the score. You would be sent email with the offer. You can either accept it or reject it.
If you reject it, there will be a next round. You would have given your choice of streams (e.g., VLSI, Comm systems etc) in order of preference while filling the application. Once you reject the offer, only if there is any seat available in your higher preference, then you would get a second offer in next round.