As the documentation mentions, in order to access files on an iOS device you need to use iTunes or similar software (iExplorer; if you have a computer with macOS 10.15 Catalina, you now use the Finder). If using iTunes:
1) Plug the iOS device in to the computer; this should launch iTunes
2) If prompted on the iOS device, allow the computer to access the files on the iOS device
3) In iTunes, click on the device button located in the top of the application. This will open up the iOS device's page
4) In the device page, click on 'File Sharing'. This will show a list of applications. Select Survey123 (you may need to scroll through the list of applications if it's not visible immediately).
5) With Survey123 selected, select the 'ArcGIS' folder in the right column and then click 'Save As'. Select a location on your computer to save the directory.
You will now have the data stored by Survey123. The sqlite file will be in ArcGIS -> My Surveys -> Databases