Sorry, this was a bit out of context.
As the other commenter assumed, it is for replacing/extending the functionality of the phone.
For example, I wanted to access the heartrate and other mi band data and it's quite easy with gadgetbridge and my own flutter app.
I tried similar apps before with SwiftUI and the Apple Watch and it was pretty much a disaster (although I have working knowledge of objective C ...). Dart came easy and flutter seems super powerful to me ( I just re-implemented my first very simple ios game in just a day. that's the objective c version https://apps.apple.com/us/app/blanks/id286883373 ). Will publish/open-source the other one, when it's polished.
It uses Android's. It works like a regular Android smartphone, you enable Developer Mode + USB debugging. Android Studio recognizes the phone and complies /runs your application.
I tried similar apps before with SwiftUI and the Apple Watch and it was pretty much a disaster (although I have working knowledge of objective C ...). Dart came easy and flutter seems super powerful to me ( I just re-implemented my first very simple ios game in just a day. that's the objective c version https://apps.apple.com/us/app/blanks/id286883373 ). Will publish/open-source the other one, when it's polished.