The process involved an on-site interview that lasted half a day.

It included four coding interviews.

Even though the interview questions were straightforward, I later heard that one of the engineering managers didn't think they had a position open for my skills. This was seriously dumbfounding. I had submitted a resume detailing my experience, so why was it necessary to spend half a day there answering interview questions when it wasn't even necessary?

It's fairly clear that unless you're a senior engineer, they will not treat candidates that well.

One of the interviewers asked me out on a date afterward. That was exciting.

Nothing out of the ordinary. It involves the usual tree traversal, string manipulation, and so on. These tasks are not too difficult if you're practiced on LeetCode or InterviewBit.