There were three rounds of Interviews. Two are technical and after that HR interview held with discussion of all the details.HR told you are selected for this position and will be sending the offer letter within a week. After then no response. HR is totally a fool who does not know for which position they are recruiting and what is going on.He does not seem to be professional behaving like illiterate person.They cant afford the candidate but they will simply call for interview. Total waste of time and energy.

3 Rounds of interview 2 Coding rounds 1 system design over multiple days. Internal Pro level proficiency is needed. Prepare via leetcode coding. Watch youtube for system design. Combined these should be good