HRM 600 automatic activation while forerunner 945 on wrist?

Hello,

I am using my forerunner 945 as my GPS recorder while cycling.
In order to get clean cardio data, I was using a Garmin cardiostrap (HRM from 10 years ago) but had to replace it because the electrodes bands were damaged (unglued).

So I bought an HRM 600. It is paired to the Garmn Connect app (5.15 update).

3 questions:
Is it normal that my Forerunner 945 can't detect the HRM (it is ON) while trying to add a new device? I have been able to pair it with my Wahoo Element.
will i need to act on my phone to launch an activity on HRM600 _or_ will the HRM600 automatically detect something? (I think the answer is yes)
what cardiac data will be recorded in my bike tour (wrist or HRM)?

Thanks for your inputs
Cheers
FG