Interview process and timelines for costco travel SWE

Hello All, I am currently interviewing for SWE role and have been invited for an onsite interview. How difficult is the onsite interview? Any tips? I have been told i have another 2 rounds that includes an interview with a director. I applied for L3 role but am curious as to why a director level discussion is needed.Is that norm? Or they are planning to hire at a higher level than intended???What should I expect in the discussion Tc 95k #interview