Hey,
For the majority of providers displayed on our integrations page, you do not require api keys as we fully manage authentication. However, for Strava, WHOOP, Inbody, Huawei and Virtuagym, you will have to apply for your own keys and submit them via our developer dashboard by going to Dashboard > Connections > provider > Edit > Provider details
The Terra API is an internet API and your users can connect their fitness wearable accounts for almost all integrations listed via a web-interface, meaning you do not have to build a mobile app to retrieve data from devices like: Fibit, Oura, Garmin, Google Fit, Strava. However, for Apple health and Samsung Health, specifically you will need to ship software for the users mobile device using one of our Mobile SDKs for which we support iOS, Android (native) and React Native, Flutter (Cross-platform)
Hope that answers your questions