What would be?

I felt all my rounds were positive.

One downside of the interview process is that candidates hardly get told _why_ they were rejected.

It is unlikely that your phone cost you the job but Apple as a company does seem to live in a bubble of "Apple is best. Apple for everything " so there's always the slim chance that you got an interviewer who lives by that and decided to not like you much. Pretty unlikely though but it shall remain one of those great mysteries of life

Actual feedback is rare and extremely valuable.

Mostly you will get "decided not to go forward", or "they felt you didn't have enough depth in <SOME SKILL>", which might or might not be true.

What recruiter is going to say, "Jeff said he could never work with anyone who would carry a cheap phone".