So I want to throw out something quickly - just because someone claims a photo has been shot on an iPhone doesn't mean that they shot it in the same way as you or I would. Lots of people have special lenses that they attach in front of the iPhone camera lens to get a macro / wide angle view. They almost always shoot RAW with an app that supports this. They also use a mini tripod to stabilize the photo and use techniques like long exposure.

https://pbs.twimg.com/media/EEH38ZxUYAQDPBT?format=jpg&name=...

There's also a lot of editing skill that goes into this - a photographer won't simply take whatever processing the phone comes up with use it out of the box.

I personally use the lightroom mobile camera to capture RAW photos and play around a bit with curves, exposure, hsl etc. to get the look I want. I assume if they're a pro photographer, they move the RAW photo to the PC / iPad and add some photoshop magic as well.

The output (depends on the photographer ofcourse but for the ones I like) looks very natural and doesn't have the same look as simply applying an instagram filter on top of an iPhone camera shot image.

Obviously the composition etc., that folks are talking about matter a lot as well but IMO, these days camera hardware is so good that you can get all sorts of detail out of what you assumed was a blown out camera RAW. Crops work fairly well too because of how high-res the photos are, so if you're not too pleased with your initial composition, crop + photoshop fixes will get you a long way. As a whole, photography these days is about 70% editing and 30% photo taking technique (lighting, composition, aperture, shutter speed, iso etc.)