Let me tell you this from a "what really happens" scenario where I work.
In the cold had light of reality, Quality is controlled fundamentally by the customer.
If the customer accepts products up to x standard then we will pass items of x standard out the door to ship.
If the customer rejects those items and wants xx standard, then we have to tighten our act up and check to a xx standard.
Whilst the factories that make the products for Apple are vastly larger, I honestly do not expect they are any different.
There will be a level or tolerance on all aspects of every Apple product, as no physical item is perfect, it just depends on how close you look and what you are looking for.
Dust, dead pixels, dents, dimensional accuracy, haptic motor strength, screen brightness, etc, etc, etc. There will be tolerances on everything, even the chips.
Surface finish is no different. You can write, blemish free, but what's blemish free? Who is going to be looking under what light and what magnification?
Some items will slip thru, that's just what happens. Someone is checking something, they get spoken to and interrupted, and they think they looked in an area they did not, and miss something.
Mostly though, esp with the surface/visual finish, it WILL come down to a judgement call.
If a factor is under amazing pressure to get, 5,000 a day out the door, there are going to be time when something will be let go that you hope will be accepted by the customer.
It's worth sending out 100 less than perfect items to your inspectors who are looking for faults, if 75% of these get accepted by customers and you have to replace 25% of them. Than it is for you to reject 100% of them.
