Thanks. I agree with your concern. Apple's support answer paints a different picture.
"Your HealthKit data, including your health records, remains under your complete control. You can choose to use iCloud to keep your data up to date across your devices, choose to share your data with a third-party app, choose to share your data with Apple under the Improve Health Records program, or choose to back up your data to an iTunes encrypted backup on your computer."
https://support.apple.com/en-us/HT209519
I interpret this to mean we maintain full control who and where your health information is stored. It's only stored on your device, unless you chose to share it with other apps, and/or use iCloud to sync across devices.
As an American, if I am being honest, I trust Apple far more than I would a Chinese company--given what I understanding about the CCP.