I imagine that people (perhaps at the VP or director level) decide that they need more scientists. Following this, I suspect that job descriptions are sent to the HR folks for posting, and coordination is made to forward incoming resumes from HR and other employees to the relevant hiring committee.
After short lists are made and phone screenings are done, I suspect that 2 or 3 candidates are brought in for interviews and then the hiring committee makes their decisions.Upon further reflection, it is my guess that the decision to hire is probably made once a year when departmental budgets are set (thus the idea that VPs/directors are involved.) Most large companies seem to have an annual push to recruit new employees that starts in the fall/winter, hiring for the summer of the following year. I assume that there is some sort of coordination between the budgeting ("we can afford X new scientists" and the recruiting ("this year, we need X new scientists.")
I'm probably giving the process short shrift, so I'd love to hear from readers who've participated in hiring in Big Pharma.