How is bangle different from other smart-watches?

Hello @Gordon, thanks for your quick answer.
I already checked the apps, in fact I even checked the code of some of them, I'm a curious person.

you'd have to write your own app (Web Bluetooth or otherwise)

This is not clear for me. Do you mean to develop an Android app or, as an alternative create a webapp that uses Web Bluetooth?

I saw the expected battery life is of one week, but is that realistic or it's just a technical specification of the manufacturer?

Thanks!