iPhone XS Max Microphones
Location of the microphones on the iPhone XS Max
The iPhone XS Max now has the ability to record in stereo thanks to the many microphones that are included. The only way to enable the stereo microphones is to enable the "Record Stereo Sound' feature in the Settings > Camera. This appears to be a feature exclusive to Apple Camera.
Not always in Stereo
In my testing I noticed that videos aren't always used stereo sound. I think this is because the iPhone determines if stereo needs to be enabled. So if I am talking in a room by myself the phone may not be enabled, but if I record the crowd at Fenway Park it will certainly be in stereo.
Third Party Apps Stereo Support
Developers haven't been able to add stereo sound support since Apple hasn't made that feature available in iOS API. Hopefully in a future update that developers will be able to use the feature available only in iPhone XS.
Where are the Microphones on the iPhone XS Max?
This diagram shows the location of the Microphones on the iPhone XS Max:
What about Audio files?
Apple's Voice Memos app is still recording sound in Mono format. If your looking for a more powerful audio capture application try GarageBand.