A:AnswerIt turns on when you turn your wrist. Be aware though if you have it monitoring your pulse ox that it glows read on your wrist. This has annoyed my husband at night since it attempts to take measurements frequently (it doesn't measure if you move at all).
A:AnswerYes, it constantly displays heart rate on the "home screen." But I don't know how it uses the data within the app when you select exercise as an activity.
A:AnswerCheck the product description for included band length. The small/medium band fits (just barely) my 7-1/4" wrist. It's tricky to put on but it is comfy. The large band purchased thru Garmin is about 30 clam shells. Not worth it too me.