AppStudio OTA web app https://appstudio-ota.arcgis.com can be used for installing your custom Survey123 app on iOS devices.
To use the AppStudio OTA Web App:
If you are looking for a way to side load the ipa offline, I've been using Xcode to install custom apps to iOS devices.
In Xcode, choose Window > Devices, or press command-shift-2. You should see your device displayed in the Devices window. Either drag your .ipa file into the “Installed Apps” section, or click on the plus sign and select your .ipa file.
Above post describes few other options for installing apps on iOS devices.
Nice one...I also want to this type of information because I recently bought the iPhone X 256 GB when I think to buy this phone I consult with the apple expert team by dialing iPhone Support number, they told me that iPhone X is better for you and told me many new things and features of this phone.