Hey,
To get into NIT you need to be a top scorer in JEE mains. On the basis of your JEE Mains score you are assigned the NIT and afterwards branch at that NIT only on the basis of that score. When it comes to IIT, first you'll have to clear JEE Mains then you'll have to clear JEE Advance with a very good score, and on the basis of JEE Advance you are assigned IIT and afterwards branches at IIT's on the basis of JEE advance score. Also to be eligible for JEE advance you must clear JEE Mains.

Hope this solves your query.

Good Luck!!