DiCon Fiberoptics Nano-Optics Process Engineer interview questions

I went through 3 rounds of interviews, 1st round was screening with the manager, which was straightforward. The manager seemed pretty nice, They then sent a folder of learning materials for me to prepare for the 'exam', I then let them know when I'm ready. 2nd round was the online 'exam' where I had to answer 5 questions on paper in an hour then send them to the recruiter. The questions were pretty simple, but did not cover all the learning materials, which I spent a lot of time on. After a week, they let me know I passed and scheduled an onsite interview. Went to the onsite interview. Hotel, food & flights were paid for. Met with the manager, visit the site and other engineers, lunch, then presented my phd work to 2 managers, Got asked lots of questions about the company products and history. Finally, met with the HR person, who told me more about salary and benefits. I tried to ask for higher salary but she said it's impossible. They told me it would take 2 weeks to respond. I left feeling semi-confident with my performance. Heard nothing after 4 weeks. Sent a follow-up email to the hiring manager and he said I got rejected. Bummer but ok. How I felt about the company. It seemed like an old company trying to get back to the market with some niche products. Everyone seemed to be pretty happy but not necessarily loves their job. Seemed like a good first job as I noticed most engineers there are very new and there are only a handful of more senior engineers.