No, of course they didn't. Apple is widely acknowledged in the industry as having amateurish design and an utter lack of anything resembling perfectionism or attention to detail.
Seriously, this may be a flop from them, but I cannot comprehend the mindset that would surmise they did no usability testing off a few screenshots.
I would love to see your source on this one, because I never stop hearing the opposite.
Yes, it does, actually. As resolution increases, so does the Nyquist frequency, which means you can accurately convey higher-frequency signals. In spatial terms, more resolution means finer lines without aliasing errors.
Going higher than that probably means thinner blank lines will just appear fainter and not thinner.
Speaking of this design, the icons are asymmetric. More so by the unharmonious colour selection. However, the notification centre and animations are well done.
A lot (many? most?) of Apple's design decisions are made by designers who create the best product, based on a combination of their intuition, design sense, and overriding design principles.
Some application (Podcast App) - have clearly never seen any form of usability testing prior to release.
I cannot believe such an App had even a single "average" user attempt to use it - it had to have been entirely the product of one or two individuals operating in a vacuum.