what is the selection process?

It depends. But they have below process for selection:
Step 1: Short listing candidates on the basis of aggregate percentage till 3rd year (it was annual pattern back then) >= 60%.

Step 2: Written test which was mostly technical with both - multiple choice objective and descriptive questions. There were few 'what if' question to gauge the aptitude. I don't remember any puzzles like the IT companies had back then.

Step 3: Group discussions on a policy or technical decision to be debated.

Step 4: Technical interview which again, was mostly technical. But a senior HR sat through it and asked couple of question regarding career ambitions.

Step 5: HR interview which now focused on personal data collection. End of this round, offer letter would be given which is subject to qualifying medical test.

Step 6: Medical test which was quite detailed than routine and people did get disqualified for minor reasons - almost military-like.