There appears to be no theoretical limit to the bit depth and sample rate an iPhone 7 can produce. The stock Apple apps and the built-in DAC used for the speakers and their headphones only support 16-bit at 44.1 kHz. That being said, devices plugged into USB/lightning port use their own DACs and iOS provides an interface in which an app (or your own code) can access the source file directly before conversion.

I can't link you directly to Apple developer resources for specific methods/classes, as I no longer have a developer account but this article seems to have a lot of the basic info, even though it's presented as more of a rant than anything else:

http://www.recode.net/2016/9/14/12904906/apple-iphone-7-audio-sound-quality-opportunity-airpods

There appears to be no theoretical limit to the bit depth and sample rate an iPhone 7 can produce. The stock Apple apps and the built-in DAC used for the speakers and their headphones only support 16-bit at 48 kHz. That being said, devices plugged into USB/lightning port use their own DACs and iOS provides an interface in which an app (or your own code) can access the source file directly before conversion.

I can't link you directly to Apple developer resources for specific methods/classes, as I no longer have a developer account but this article seems to have a lot of the basic info, even though it's presented as more of a rant than anything else:

http://www.recode.net/2016/9/14/12904906/apple-iphone-7-audio-sound-quality-opportunity-airpods