Most of the time changes will only be in the www folder. However some project changes may affect the Xcode project too, such as changing project settings, changing export settings, or adding/changing/removing third-party addons that use Cordova plugins. So for basic changes updating the www folder is probably fine; if you're not sure, a diff tool can easily show the difference between two folders of files to compare two exports, e.g. WinMerge on Windows which I use myself sometimes (although I'm not sure what macOS equivalents there are).
Remote Preview is designed to let you quickly test on mobile without needing to export every time. Is there a reason you're not previewing with that for faster iterations?
On a side note, upon searching, it seems like most documentation on Construct -> iOS is about 5+ years old.
AFIAK the process for exporting to iOS hasn't changed substantially over the past few years, so the existing documentation should still apply. If anything is out of date then let us know.