Elementum Software Engineer-Backend interview questions

I interviewed at Elementum for "Software Engineer-Backend" position. My interview experience at Elementum was very good. They got a responsive HR team who communicated details well ahead and kept me in the loop whenever there are changes in the timing. Technical interview process includes intro talk (get to know each other) with HR, followed by coding interview using hacker-rank. Then I was called for a half-day long (very usual these days in any company) onsite interview. I had 5 interview rounds which were as follows: 1. Programming Test: I was given a question and asked to program with an IDE in a mac laptop. Duration was 45 mins. It was followed by code-review by 2 member panel. 2. Data Structures and Programming: It was a 2 member interview panel. They asked questions on Data Structures and its equivalent java implementation. How, when and why do you choose a particular data structure over another. Then there were two white board programming questions involving Lists, BST, Binary Trees. 3. Design Patterns and Architecture: It was a 3 member panel. They asked me questions on multi-threading, patterns like Singleton, Observer etc. They asked me to design a model similar to supply-chain issues. 4.Meeting with COO. 5.Technical interview with two engineers. Finally, I had a phone conversation with the hiring manager. Interviewers are very open and they were ready to answer any questions about the work culture, company, management etc. Two days later, I got the offer. I accepted it.