Zillow Software Engineer Internship interview questions

I had my first round interview around three weeks after applying. It was about 5-10 minutes of behavioral questions (correlated to their core values) followed by 35-40 minutes of technical questions. The interview was conducted by one of their software engineers. I moved onto the second (and final) round after that, which was a little more than two weeks after the first round interview. The process was the same as the first round, but there were two back-to-back interviews instead of just one. I thought I did pretty well. I was able to answer their technical questions and I was able to provide more information regarding my past experiences in the final round after my first round interviewer said I should have talked more about my relevant experiences (I asked him for feedback). I asked for feedback for my final round interview as well, but they said they would send out feedback a week after my interview, which I guess is done with all of their candidates. However, I was rejected almost in an instant - I received a rejection email the morning after the interviewers reconvened. I was baffled, surprised, and upset. I had no idea what I had done wrong. They said their email that they can't provide me the specific feedback that they said they would send earlier. So I still have no idea why I was rejected. I don't know how I should improve from the mistakes I didn't know I made during my final round of interviews. The most frustrating part is that they are unwilling to share why, which goes against one of their leadership principles of increasing transparency. How can I improve my interview skills to actually get the position next year? How am I supposed to improve without knowing where I went wrong, especially since I thought I did well? Transparency would be appreciated.