Gadgetbridge is an Android companion app not only for smart bands and watches but also for wireless earplugs, multimeters... and lately also for the Flipper Zero.
Unfortunately, Fitbit is not (yet?) supported by Gadgetbridge.
Support for other smart bands like Miband/Amazfit is pretty good, the Fossil Hybrid HR is now almost like we wanted the Pebble to be. Someone has been working on support for the Withings Steel HR hybrid watches, which is also really nice.
See our wiki, here is a good place to start: https://codeberg.org/Freeyourgadget/Gadgetbridge/wiki/FAQ