Operating Systems

Hello Tejas,
It is the question for which only you can answer for. It depends on your interest that in which you have more interest. As you have completed the B.Sc. Degree, it would be better to go for the M.Sc. Degree as it will be of your subject.
If you have more interest in computer, coding or designing then you can go for the MCA or other related courses. If you are interested do M.Sc. Of higher studies from IITs, IISc, NITs and other top institutes, than you can should apply for the IIT JAM exam.
The IIT JAM exam is organized for the admission into M.Sc. Ph.D. M.Sc-PhD and other integrated courses from IITs and IISc institutes of India. Know the complete details for the exam from official website or https://scoop.eduncle.com/iit-jam-eligibility and make sure that you have read all the instructions carefully. The minimum eligibility to apply for the JAM exam is graduation degree with your subject for more than 6 sem or 3 years.
If you want to pursue MSc or higher education, then it will be good to go for JAM exam but if you are interested in coding, you should go for the android development.
The UI/UX is totally different field. you must have deep knowledge of the editing software and designing.
The PhD will allow you to explore your post graduation and subject, The master's will allow you to stay connected with your subjects.
Hope it will help you. All the best!

I started learning Android through YouTube tutorials. It took me almost a year (being an IT student) to create a decent app for Play Store, you can take more or less time depending on your learning capabilities and the amount of time you put into learning on daily basis. You don't need to know everything about Android Studio and Java to create Android Applications. Just watch some tutorials, start playing around with Android Studio, create some sample projects, and start with text views, buttons and you'll begin to understand what's happening with the code. This is the way I learned it. Honestly, I don't know complete android, I learnt like 1 or 2% from those YouTube tutorials but still I am able to create some applications. I have uploaded like 5 of them on Play Store, where one of them received 50k+ installs within 2 years of release. So, start learning and practicing without worrying about the time consumption, it's always fun to code and the feeling of being able to create apps for others to use is just awesome.