Dear Shivraj
Im going to make this very straight and simple.

If youre good enough to make it through the integrated MS program at IISER in any discipline, Im certain youd have developed a niche for fundamental or even applied research by the time you graduate. If Im not wrong, youd even end up with three or four quality publications as well.

Once youre in this kind of a zone, students primarily opt for a career in research and prefer going in for a PhD to get their education formal and complete. You do a PhD, sharpen your research skills, do a postdoc, and then join a central research facility to continue the work you started off either during your MS or PhD.

Having said that, its not imperative to get into research once you graduate from an IISER. Depending on the discipline you graduate, you stand a pretty good chance getting into industry. Pharma companies, lasers, optics and photonics instrumentation companies, biotech firms, etc. are a few core areas students tend to focus on industrially.

There are also several financial consulting companies that hire top brass graduates, irrespective of the course, solely because of their analytical skills and that is something an institute like IISER helps you develop over the years.

But then, apart from the select few, its usually rare to find IISER grads NOT get into a PhD program some time later.

All the best.