IMO the key thing is that interviews have a heavy component of randomness.

The question your interviewer selects (both topic and difficulty), how the interview performs as far as presenting the question/giving hints, how the interviewer is feeling, any biases the interviewer might have, and so on.

You can of course 'increase your surface area for luck' by studying the questions/topics a company tends to ask, being an interviewee who comes off as well qualified.

But, if you get an interviewer who is in a bad mood, if your interview isn't competent or paying attention, etc. then you're just screwed.

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I say this because the OP was honest and failed; you were honest and were hired. There's no 'secret' to getting in. It is a combination of:

1. Being a competent engineer (or student)

2. Putting in the time to study, leetcode, practice, etc.

3. Luck