does one should qualify jee adv in order to get into nits otherwise mains is enough for it

HI, Through Jee Mains you get admission in IIIT and NIT's, other Centrally Funded Technical Institutes (CFTIs) and engineering colleges of the participating states.
The JEE Main is also the eligibility test that allows students to give the JEE Advanced exam which eventually leads to admission in either the IITs or the Indian School of Mines (ISM).

So to get into NIT, JEE Mains is enough.
Good Luck.