There is nothing like just clearing JEE Mains. I have seen instances in which candidates with very low marks get admitted to very good NITs.
It is a selection test in which you are offered seat in a college. If no one opts for a seat offered, it comes out in second counseling and then third.
This way if no one with very good marks applies for the seat and decides something else, the lower marks carrying candidate gets a chance to apply for the same seat. No college wants to keep a seat vacant and thus fills up the seat.
P.S. Salute Indian Army. They keep a seat or post vacant but don’t offer it to any non-deserving candidate.