When exercising, you use the workout app.
When at rest, the watch records your resting heart rate (which is a health function rather than a fitness function).
The watch also monitors how much you have exerted yourself during the day. You can set target for movement which is monitored in terms of calorie burn. The calorie burn calculations use heart rate data and regardless of whether you have used the workout app or not, a record of exercise gets added to your exercise record for the day, whenever you have an elevated heart rate. The heart rate measurements are taken less frequently when not in the workout app.
Yesterday I went for a cycle ride on a course I cycle often. I started the workout app and paused the workout part way through when I was waiting to meet a co-rider, but after we met, I forgot to unpause the app. It was a good daily session (about 45k), so I was annoyed when I realised I had failed to resume the workout app, assuming the exercise session would have failed to register against my daily target. When I checked the exercise app, the recorded calorie output was, as I expected, far lower than it should have been (about 550 calories when normally over that course it would have been about 1500+). However when I checked my daily exercise record, I was pleased to see my energy output had been recorded (so I still exceeded my daily target). Over 1200 calories had been added to my tracked exercise for the day. So this confirms the watch tracks exercise even when not in the workout app. However I noticed the amount added was about 20% less than it would normally have been if I had used the exercise app throughout the course.
So, to me it is clear the behaviour of the watch is:
1. Any elevated heart rate is tracked as exercise and added to your exercise record. If you are not specifically in the workout app, the sampling rate is clearly lower. It may take a while before the watch registers that you have an elevated heart rate. And once it decides you are exercising, my guess is that it does so less accurately (with a lower sample rate) than if you specifically start the workout app. It makes a record of your energy output for exercise tracking and checking if you are meeting daily targets (if you want that function), but, though it checks your heart rate every now and then, doesn't keep a record of your heart rate when it is elevated.
2. Switch on the exercise app and the tracking of energy output is more accurate and done with a higher sample rate for your heart rate. Plus it keeps a record of your heart-rate throughout.
4. When not in the exercise app, though the watch checks your heart rate, it only takes a record of your resting heart rate. I presume this behaviour is deliberate and is a health monitoring function (e.g. specifically not an exercise record). Resting heart rate is a significant health metric. Lower resting heart rate indicates a higher general fitness.