My experience:
- Using Xcode 10.2.1 on iMac 10.14.4
- Iphone X with iOS 12+ connected via USB
- Download xcode project from C3 (cordova with xcode option)
- Change Xcode Project Setting to 'legacy build'
- Many warning about deprecations, etc. on loading C3 project into xcode.
- Add my provisioning to the project
- Clicked run and C3 app appeared on my iPhone.
Some caveats - I find that generally, effects don't always work well on iPhone, sometimes perf issues, sometimes issues if you are using parameters in effects that require knowledge of true screen resolution (depending on scaling modes, the retina display can be running at 2X of reported resolutions.)