In a related question, does anyone know of any monitoring product (e.g. FitBit or whatever) that does NOT send your data to a centralized server outside of your control?
As far as I know the Apple Watch is the only one that doesn't sync to a server somewhere. All data is kept stored in HealthKit on your device.
We have servers that store the raw data and run the algorithms on it, and then it's also synced to Google Fit (on Android) and HealthKit (iOS).