Physics

Yes, you can do that. After B.Tech, you are eligible for M.Sc. Entrance exams like IIT JAM. There are many students who do M.Sc. After B.Tech. You can do M.Sc. in maths also. Many people are dual master holders. So you can also do that.

To get admission in M.Sc. Physics in any government college you need to give entrance exams like IIT JAM JEST Du entrance exam, BHU entrance exams and many others depending upon the college's. Fees structure vary from college to college, for example in IIT Delhi it's 30,000 thousands rupees for one semester. You need more than 5.5 CGPA or 55 recent to be able to take admission. It's the minimum percentage required for any entrance exam then only you can take admission
Hope you got your answer.

Yes you are eligible for most of the colleges in Delhi University if you are having a good BFS percentage.