Writing Statement of Purpose As a Bachelor Student (Physics) I'm a third-year bachelor of a science student. I'm shortlisted for an interview for the Integrated Ph.D. course. I'm Physics major student. It's asked by the institution for a Statement of Purpose.

One-page original research write-up by the candidate (research statement of purpose).

I have never written a statement of purpose in my whole life and this is the first one. Now after searching for what it is. I found several posts which help me a lot and the most important one is this one. If I summarize the points given in the answer as

What have you already done?
What are you working on now?
What might you want to work on in the future?
How does my department fit your research goals?

Well, this is it, and for me, it's quite satisfactory.
Now the shortcomings as a bachelor's student are that we are not exposed to real research. We just get an introductory idea of different branches of physics. Now the problem is, As a bachelor student, I haven't done any research work in the past (I did an intern on Astrophysics but it was based on introductory knowledge) nor I'm currently working on any project and apart from a little glimpse, I have not a big picture of what I'm going to do in future. I know I want to be a particle physicist but I don't know what specific thing in particle physics, I want to do work in and It might possible that the interest might divert to some other path, say Astrophysics.
This eliminates the first three points. The fourth one, again, I can only say, the department itself help me decide what I need to do in future. It gives me an understanding of the subject and so on.
So It seems there Isn't much to say in the Statement of Purpose as a bachelor student. So I want advice on How should I write my statement of purpose?
Please ask any information you need about me if necessary.

I'm not studying in United State. I'm a student from India. And I don't understand what's common between this and

How does the admissions process work for Ph.D. programs in the US, particularly for weak or borderline students?
SOPs are a rather dreaded part of the application process for many students in the tagged geography, precisely because there is a big gap between the expectations and method of working that students are typically used to. The previous comments and answer are indeed relevant. Given your current situation, I'll add some specific things that you can think about, leaning towards pragmatism rather than the ideal.
(1) The SOP is essentially a narrative that has to sell. (Arguably so is most research, but that's another tale). You must know your audience. Don't make one boilerplate SOP for all the departments you apply to. Visit their websites, read up what they work on. Try to use social media to speak to current students. You may not know what you want to work on, but you must know with reasonable veracity what they work on. Don't start writing your SOP without doing this. The underlying benefit (arguably the real reason) is that you will me more updated about what is currently happening in the field now, and this will help you cultivate your interests more sensibly.
(2) Treat each SOP as an independent and exclusive document. Once you've identified the department &amp; professor/group you would like to work with the most, think of everything you've heard, learnt or done that may be relevant to the topic. Oftentimes this is a research project, sometimes it is a class that really interested you and motivated you to read up further. Sometimes it is even a personal experience or anecdote that left a deep impact on you, inspiring you to move towards the respective field. You need not have done concrete research work on it, but you should be able to demonstrate some interest or passion, or even a convincing motivation.
If you are not able to find yourself motivated by a certain field, it may be a strong indication to look elsewhere - one more underlying benefit.
(3) Connect your background (educational, personal if it is relevant), your motivation/experience/interest and relevant achievements to the field of interest. Avoid irrelevant history. The badminton championship you participated in is unlikely to be helpful, unless you are using it to make a point about striving for excellence. Or being fascinated by the aerodynamics of a weighted shuttlecock. I hope you catch my drift here.
(4) Take seriously the last segments, which describe how the university could help you achieve your goals, and equally, why you are a good fit for the university. Be very specific about the first. You should be able to demonstrate that you know what the department/group is known for, what sets them apart, what sort of impact they have. Use details if possible. If you've been able to speak to some students there, mention it (show your efforts and initiative). As to why you are a good fit, describe your attributes and qualities; but ensure that they relate to the personal history/experiences/achievements that you previously mentioned. This adds continuity, keeps you honest and prevents the SOP from reading like a boilerplate job application.
Take your time with this, and remember to respect the reader. Its a big opportunity for you and an important investment for the group/department that you join. The worst thing that you could do is treat it flippantly and churn out something that doesn't show serious thought.