Having said that, it seems that the MP4 videos are generally encoded in a way that most things are capable of understanding. So it would seem odd that iOS devices would change that playing field. Unless, of course, Apple wants to goad their customers into paying even more than they do for special things that allow them to consume generic type data on their devices.
Cheers... Rick :)
Ideally, you want the chosen format to create high quality videos with small file sizes.
There is a trade off between file size and quality.
Some formats create higher quality videos and support smaller file sizes by comparison.
I am not an iPhone user. Left Apple after the Apple ][+ because of the new "closed architecture" ideology (i.e. we will dictate what you can do on our computers.). However, a quick search
The following suggests MP4 is supported in most iPhones.
You have to read the next article. It starts of stating only MKV AVI FLV are played natively but later states the iPhone 7 Plus can play some MP4 videos if they are H.264 or MPEG-4. Apple will allow you to use all approved formats.