

Well on iOS there’s the Apple health app. To my knowledge it stores health data locally. I’ll double check now.
Edit: it does store health data in iCloud by default, but according to Apple its end to end encrypted
By default, iCloud automatically keeps your Health app data, including health records, up to date across your devices. To disable this feature, open iCloud settings and turn off Health. iCloud protects your health records data by encrypting it both in storage and during transmission. If you’re using iOS 12 or later and have turned on two-factor authentication for your Apple Account, health records are encrypted using end-to-end encryption through iCloud. This means only you can access this information, and only on devices where you’re signed in to iCloud. No one else, not even Apple, can access end-to-end encrypted information.
Yeah I know it’s not FOSS in the slightest, but it’s not a predatory app selling your health data to the highest bidder, presumably. I acknowledge that requires taking Apple’s word at face value though