I have a Fitbit in a simple, non-watch form factor. I use it ostensibly to track only two things: steps and heart rate.
Heaven help me in trying to figure out how to get its data to Apple Health. I must sync through Google Health. A third party app is needed to sync from there to Apple Health. Most (all but one as far as I can tell*) apps that do the sync transfer step counts as the average over the day spread over each of the 24 hours. Today you do 12,000 steps. Translated after sync, you did 500 steps early this morning between 2-3am. And then, as a kicker, the additional 500 steps on each hour are mostly added back to the All Devices step count in Google Health, which then gets pushed back to the Fitbit. Viola, your Fitbit now reads some 2-3 thousand more steps just because you synced it!
Long story short: If you intend to track your full health portfolio with any seriousness, you can only do so without possible manual convolutions when you stay exclusively in the Google Health or exclusively in the Apple Health ecosystem. Nary the twine shall meet (without your careful oversight). Consider purchases of health-tracking hardware (Apple, Garmin, Fitbit, …) accordingly.
[* one app that does appear to sync Fitbit steps properly by time – Health Sync – has serious UI glitches in starting up]
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JJW