how to study defense technology in India after B.Tech

You can go for Defence Technology at Defence Institute of Advanced Technology, Pune after your BTech.

For admissions to the M.tech course at the Defense Advanced Institute of Technology, Pune, you must give GATE and get the required cutoff. The GATE test results were recently released. The Pune Advanced Technology Defense Institute has not yet started the admissions process and the dates are not yet mentioned. So keep an eye on the official website for the latest information. The DIAT institute offers postgraduate courses with specializations in Guided Missiles, Radar Communication, Artificial Intelligence and other areas. The entrance exam taken for these 2-year courses is GATE.

You can visit the link below to know more about DIAT, Pune:

https://www.careers360.com/university/defence-institute-of-advanced-technology-pune

I hope this answer helps. All the very best for your future endeavors!