iOS devices do not report their model in their User Agent string but it is possible to make some assumptions based on the reported screen resolution. This table summarises the screen resolution and the associated iPhone.
|iPhone <= 4||320 x 480|
|iPhone 5/SE||320 x 568|
|iPhone 6-8||375 x 667|
|iPhone 6-8 Plus||414 x 736|
|iPhone X/XS||375 x 812|
|iPhone XS Plus||414 x 896|
Note that the iPhone 6 with UI zoom enabled will report the same resolution as the iPhone 5 and, likewise, the iPhone 6 Plus with UI zoom will report the same as the iPhone 6.
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