Science

Hello Animesh, it's very nice to hear from you that you are interested in doing your post graduation in aerospace engineering branch. If your research B.Sc. Is of four years duration then you are eligible to appear for GATE which is the entrance exam for admission into PG in engineering in different branches. Do not worry. Even after B.Sc. Research, you can appear for campus selection in your institution. My sincere advise would be to continue your study in Science stream by doing PG and then doctorate. There is quite huge demand in market. To help you I am attaching a link that will give you all the required information about different aspects of eligibility for GATE 2023. https://www.shiksha.com/engineering/articles/gate-2023-exam-date-application-form-eligibility-blogId-86267#:~:text=GATE 2023 Eligibility Criteria, undergraduate degree are also eligible. Best wishes to you.

Yes in some institutes and No in some institutes, it's possible to study M.Sc. In Physics after B.Tech in Mechanical Engineering. But only few colleges are giving chance for B.Tech students like Delhi University. Do some research about colleges giving this chance and join accordingly.
You would only be accepted to a few institutions if you wanted to pursue an MSc in physics. Traditional universities will not even accept you for a master's degree in physics. Furthermore, btech mechanical engineering is solely based on basic classical mechanics or lower level classical mechanics, the majority of which is newtonian mechanics.
BSc physics is focused on quantum mechanics, and almost all areas of study in BSc physics use quantum mechanics like there isn't any newtonian mechanics in the universe, so whatever they would be thinking in India about degree equivalence, a BSc physics is way higher than a btech mechanical engineering in terms of theoretical knowledge and depth of physics and mathematics, in fact, a BSc physics is way higher than btech, So, if you get into one of those select few universities, you'll have to work extremely hard or you'll end up in hell. Also, if you want to study physics overseas, a btech is ineligible for an MSc or MS in physics. Only those with a BSc/BS in physics are eligible. As a result, you won't be able to pursue an MSc in another country because their qualifying standards only allow for BSc/BS physics.

You can do Masters in psychology being a B.Tech CSE graduate. It's quite easier to switch now to Masters psychology. Get ready to do hardwork since you are not a psychology background student. Few colleges won't allow non psychology background students, check before and join accordingly!
Without a bachelor's degree in psychology, you can pursue an MA or MSc in psychology. The eligibility requirements will differ from one university to the other. Many leading colleges provide an MA in Psychology to graduates with a 50 percent graduation rate. So, if you have a B.Tech with a grade point average of greater than 50%, you can apply.
Some colleges grant admission to courses based on their degree grades, while others grant admission based on their entrance exam rank. In addition to the written test, some universities will conduct an oral examination. Almost all major institutions offer this subject, which you may research on the university's website.