If user collects data in Survey123 for iOS in offline environment, where is the data stored? If sync (data submission) fails for some reason, is there any way to retrieve data collection form iOS device?
You can choose to send your survey later after completing it, which will transfer it to an outbox--and it will do that automatically if your device doesn't have an internet connection.
I just checked how Survey123 handles disconnecting while sending data, and the app does hang until you shut it down. But the survey it failed to send goes to the outbox, and can be sent from there when you reopen the app.