Hey,
For iOS (Apple Health), the data is only accessible via our Mobile SDK, there's no cloud API from Apple, so a native app is required. However, Terra's SDK makes it straightforward to integrate: you embed a few lines of code into a minimal iOS app and the SDK handles permissions, data extraction, and upload.
If you don't have an app yet, you can also use Terra's Avengers app to connect users to Apple.
For a short research study, another option is to have participants connect wearables that have cloud-based OAuth (Garmin, Fitbit, Oura, Withings +100 more etc.) via the Terra Widget, no app needed, just a web link. If your participants use any of those devices, this would be the simplest path.