Exam

If you are in final year of your graduation then you can apply for the same.

FOR MCOM -Exams: BHU PET, FEE AROUND RS 5500
FOR BSC-Exams: BHU UET, FEE AROUND RS 8000

Yes, but you have to write JEE Mains.

First Year Fee-
For SC Candidates: Rs 9500
For ST Candidates: Rs 49350
OTHERS ; RS 92,000

Surely you can take admission, you should take DCET exam.

Yes, you can apply for AFCAT in your final graduation year.

Just go through this link and fill out the required details:
https://www.shiksha.com/college-predictor

As per the eligibility for M.Tech, you must have done either Btech or (Bsc+Msc), then only you can apply for M.Tech program. Since you have done B.Sc in Biotechnology, you further need to pursue Msc in any specialisation of Biotechnology
Yes, you can do m.tech after b.sc in maths. sit for gate, score good and get admission to the college of your choice.