Preferred Networks interview questions

I had an online assesment on hackerRank, 3 questions(easy + easy + medium), all passed. Then I had two rounds of interviews with engineers, where they asked me to prepare slides, and interviewers asked quesions deeply diving into the details.

A relatively easy programming assignment was distributed, followed by several rounds of online interview. The first interview is very generic without mentioning specific technical details peoples there are working on.