I read here that:

Apple began iPhone 6S and iPhone 6S Plus mass production in mid August.

I assume that's somewhere at the end of manufacturing process they need to install operating system on the device. It's widely anticipated that new iPhones will come out with iOS9 preinstalled. How can Apple install that new OS on those devices manufactured in August if iOS9 is still not ready yet? Or maybe iOS9 is already finalized and they are just not letting us upgrade older devices to it yet? Or maybe somewhere, as a part of the activation process, a new iPhone downloads patches for the not-fully completed iOS version that was preinstalled on the device?

PS I realize that this is not unique to iPhone 6S and iOS9 and it must happen every year with a new device and new iOS version.

  • I'm voting to close this question as off-topic because we focus on practical questions for users of Apple technology. We aren't a good place to ask why or how Apple operates. Unless there's some component to solving a problem you face, this is best in Ask Different Chat. We can help in Ask Different Meta if you need assistance editing this to describe in detail the problem you face and the research you have done to try and solve it yourself. – bmike Sep 6 '15 at 13:49

Complete speculation - however...
'beginning mass-production' does not necessarily mean building the first complete, packaged device on that day.

Presumably different suppliers will start making circuit-boards, adding components, machining the metalwork, glassware, etc etc.

All this may have a lead time of a week to 2 months.

At some point, all the critical components will come together in one place & assembly can begin.
After assembly, then they are then ready to add an OS.

That might only be happening this week, or maybe not even until next week - & iOS 9 is very likely to be ready by the time that happens.
Traditionally, the GM is released to developers within hours of the Keynote speech - which will be next Tuesday, 9th Sept 2015

It's anyone's guess as to how far in advance of that the manufacturers may get it [if at all], but we have to assume that it is sufficiently ahead of time for that gathering stockpile of completed phones to be initted, packaged & shipped in time for the release date; which will be announced on Tuesday.

They've done this before, we have to assume they're well-practised at being able to hit that date, & that the production line is designed to be run on that method.

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