You get to talk with the Principal Engineer first in a 1-on-1 interview. This was a very friendly and casual conversation. Just had to share what you know. Second interview would be a panel interview of about 2-4 seniors. Again, very friendly and casual conversation. You can laugh, share similar experiences, etc. Final interview is a 1-on-1 with a Product Manager/Director. This would be the mystery box as you wouldn't know if you pass or fail. The interviewer knows a lot based on books and not actual experience, hence, may disagree with you or find your answers not correct.