Hello Nischay,

First thing to keep in mind is that percentile and percentage are two different terms. You get scored in 12th in percentage while in JEE mains you get percentile.

Now addressing your issues; the eligibility for NITs IIITs GFTIs is based on your 12 percentage and you need to see your overall percentage for that. So as you say 84 percentage is your marks in 12 th so you are clearly eligible for these colleges after you have taken the JEE mains exam. 

Now talking about eligibility for JEE advanced in terms of percentile you have secured in JEE mains.  Here are the category wise cutoff for qualifying to JEE advanced in terms of percentile:

• General category -89.7548849
• General ews-Crl :78.2174860
• Obc-ncL:74.3166557
• Sc: 54. 0128155
• St: 44.3345172
• Pwd: 0.11371730

So if your JEE mains percentile is above the cutoff percentile of your category you are eligible for JEE advanced. 

I hope I have made things clear. 

All the best.