Apple does not report the model of the device in the user agent string. You could guess based on screen resolution, and build a classification. You could also use the version of iOS to approximate the age of the device.
Screen Resolution will also not help here as that will also be the same for all the devices.
To summarize, Adobe Analytics use the user-agent that is sent in the hit to determine the Device, Screen Size and other device specific dimensions with the help of Device Atlas. For Apple devices, this becomes an issue, since it's default browser "Safari" doesn't provide information to identify different hardware in the user-agent.
Therefore, all versions of the iPhone are tracked as one device. So for screen size as well, we set only one screen size (generally the first one in a month).
However, there can be times you see other device versions (screen sizes as well) of the iPhone. This is because the the user-agent issue is limited to safari browser in Apple devices. If the user access the website through a different browser, we can capture the details and show it in the reports.
To add to pradyut.c's query, there doesn't seem to be a solution available for traffic coming from iOS devices which used Safari browser.
If you are tracking a web property, then this is correct. The Mobile Device dimension will list the model breakdown for Android devices and just Apple iPhone and Apple iPad for those devices. If you are tracking an app via the SDK, the Device Name dimension will break down the Apple devices by their model, i.e. iPhone8,1 or iPhone10,2.