Hi Giorgio!
I have a Garmin Forerunner 935. I referred to Outdoor Gear Lab when looking at reviews.
It will detect ‘intensity’ minutes, and sometimes it does a decent job of categorising the activity. Walks, runs, and swims I’ve seen it detect without me explicitly starting an activity.
The 935 will be reporting your heart rate throughout the time you wear it. It does offer the ability to measure your HRV, but this requires 3 minutes of your time, and for you to wear your HRM. I have Garmin Connect reporting to Apple Health, so I can see trends over time.
935 can be used instead of a bike computer, but if you have a (newer) Edge device, you can set the watch to work as an extended display for the bike computer.