Hello Jonas
I can only agree with Tiger's answer

The Scanwatch would also work without a constant connection, but that makes little sense. Once the watch is paired with the smartphone via Bluetooth, communication between the app (HealthMate) and the watch runs automatically. This does not mean that the Bluetooth connection is used all the time, but in the background the data recorded by the watch is synchronised with the app from time to time. If you switch off Bluetooth on your smartphone, nothing happens at first. If you later switch Bluetooth back on, the watch communicates with the app again and synchronises the data again. However, it is better not to disconnect the watch, because after a new pairing, the settings in the app have to be made again.
On the second question: Sure, a smartphone can be connected to several Bluetooth devices at the same time and they all communicate with the smartphone at the same time.
Typical application: jogging with Bluetooth headphones and the Withing watch. The watch records the steps, pulse, distance etc. and synchronises this continuously with the app, at the same time I listen to music via Spotify through the headphones.
Hope this helps.