How do I scroll left or right in a text input field on my iPhone?
If you're using an iPhone with Force Touch you can force press down anywhere on the keyboard to bring up the typing cursor. If you drag your finger to the right/left side of the display(without letting go) the text will scroll in that direction.

Not 100% on how you would do this on an iPhone 6 or older as it has been a long time since I have used one. You might still be able to hold down on the keyboard..?