The iPhone certainly does NOT have focal lengths of 26 or 52 mm, which are implausible numbers. Because the camera body is only 7.7 mm thick, how could it contain a 26 or 52 mm lens? And also phone sensors are tiny, requiring very short focal lengths to see a normal field width.

The 26 or 52 mm numbers are 35 mm Equivalent focal lengths, and is about the lens that a 35 mm film camera would have to use to see the same field of view as the phone. That Equivalent focal length is a standard specification we often see, but it is about the lens on the 35 mm camera, and is NOT about the lens on the iPhone. It just says both have the same field of view. That is very meaningful to the many users with years of 35 mm film experience, but is about useless info if 35 mm film has no interest. Still, some small cameras do spec it that way. Apple specs say almost nothing about the camera, just f/1.4 and 12 megapixels. No info about sensor size either.

The image Exif data in a photo taken by it will show the focal length. For an iPhone XR camera, it shows FocalLength 4.25 mm, which is a very reasonable number. XR only has the wide angle camera. The Exif also shows FocalLengthin35mmFilm 26 mm. If any trouble seeing that in the Exif, the free photo viewer Irfanview will show it, at its menu Image - Information, and then the Exif button there.
Or the free ExifTool is the classic Exif answer, but may be less suitable for a computer novice (see my site https://www.scantips.com/lights/exif.html for info).

A couple of other sources show the focal lengths for the XS, at
https://qz.com/1395779/the-amazing-sorcery-of-the-iphones-newest-cameras/
https://blog.halide.cam/iphone-xs-vs-iphone-x-the-camera-hardware-changes-496b40ceae3a

And they say (presumably from their Exif) that the XS is 4.25 mm Wide and they both add the 6 mm Tele focal lengths, and they also show the numbers 26 and 52 mm that are the 35 mm Equivalent Focal Lengths. The Exif should show all of those numbers, but my XR does not have the 6 mm camera.

These are always slightly rounded numbers, but sensor size dimensions can be computed from crop factor. So the numbers make Crop Factors be 26/4.25 = 6.117 crop Wide and 52/6 = 8.667 crop Tele, which is two rather different sensor sizes. So that raises a question, since Apple says both are 12 megapixels (however pitch and sensor size is not specified, so they could be different sizes?) But the crop math is correct, and Apple Exif does specify all those mm numbers just mentioned, so I choose to believe them.

That makes Field of View of the 4.25 mm Wide lens (the so called "26 mm Equivalent") on a 6.117 Crop sensor compute to be 67.3 x 53.06 DEGREE Field of View (for the 4:3 photo aspect ratio). That would also compute Field of View dimensions of 5.3 x 4 feet at 4 feet distance. My sites FOV calculator at https://www.scantips.com/lights/fieldofview.html will compute all of this with with option 3 and 6.117 Crop factor and 4:3 Aspect, along with 4.25 mm focal length (and will also compute sensor sizes in mm accordingly).

The way to verify the field of view for unknowns is to simply lay out a 6 foot tape measure horizontally across the visible field, at a measured distance from camera (that will show less than full length of tape), and take its picture. A further distance is better than closer if you can still read it. If the lens zooms, that is another complication.

Then if that picture captures say 64 inches of tape in the overall width at 48 inches of distance, then that angle of view is 2 x arc tan (48 x 64/2) == 66 degrees of width.