NIT

If you have done your 12th and with science and mathematics then you can pursue BCA from IITs, NITs, and NIIT. If you have done your 12th with any other stream then you are not eligible for BCA from IITs and NITs, but you can pursue it from NIIT.
After the completion of BCA if you are from science with mathematics stream then you are eligible for MCA entrance exam, otherwise not, so kindly, think all these things and then take admission anywhere.

Hello Atul,
This year the system for deciding AIR has undergone changes. Marks obtained in 12th will not be counted. Marks scored only in JEE Mains are going to be considered. Secondly, there is a huge variation in ranking required to get admission in different NITs. Don't worry about that aspect now. Concentrate on studying.

For NIT college, JEE required score starts from 8 thousand to 12 thousand it may change according to the cut off or choice of your branch.

Yes, if you get the required rank for NIT college then clear JEE Advanced, after that you will get the college.

It is not like that, the scope of engineering will never get down, with increase in technology scope increases eventually. It is always good to do B.Tech from NITs.

Some of the best options comprise:
BITS Pilani
PSG College of Technology
Sardar Vallabh Bhai Patel Institute of Technology
VIT
SRM University
LPU

You need to get 90% in general category.

BITS - Pilani is the best non IIT/NIT University for B.Tech in mechanical engineering with placement records equivalent to legacy IITs.

Can you confirm the branch you want to compare among NSIT and NIT?