I don't use the device any more, but I can remember certain things. Maybe that's a good starting point for you.

Mi Band transmits data via Bluetooth. I used the unofficial app Notify for Mi Band at the time: https://play.google.com/store.... It was able to retrieve the data without having the official app installed, but you had to get a corresponding API key (of the device?).

This means that it is known that the data can be accessed in other ways and that it might be possible to find the knowledge on the internet.

A quick search on github led to this article via a project where someone interacted with an older device via Linux: https://medium.com/@a.nikisha.... Since the app mentioned above was able to read data from this generation of devices on its own and configure the device, I assume that you should be able to find something about it.

I would say good luck and check github a bit more thoroughly.