The firmware on the watch itself isn't open source. For privacy reasons that is probably of no concern. The watch only talks via bluetooth to the phone. And Gadgetbridge on the phone is free software. It only stores the data on the phone and doesn't send it to any cloud. I only have the old MiBand 4. The fitness tracking functions are a bit basic, I don't really know how how far they have improved since. But probably more something you'd expect from a $40 device and not a $100+ smart watch.

Gadgetbridge (in case you have an Android phone) is compatible with a few more devices:

• https://gadgetbridge.org/

One caveat: The MiBands need to be paired once with the official App. It's a bit cumbersome and complicated. You can delete the App after you've done it once. But you should look at the Gadgetbridge wiki and see if you're comfortable doing that complicated process.