I had a first round phone screen with HR before being cut. My interviewer was very friendly, but I could tell they were looking for something specific. My impression is that they think of themselves as the "Google" of AdTech. Very selective, very protective of their culture. If you didn't go to a great school or possess some kind of wow factor, you're probably best off applying elsewhere.

Multiple stages; first with HR, then with hiring manager and subsequently with members of the team. HR was excellent and provided a good overview of the process, the structure of the teams and their roles and what to expect during the interview.