Active Calorie Calculation

I can perhaps solve one part of the puzzle.

As I have understood it, BMR is per definition the consumption for a person who does absolutely nothing except staying alive. As soon as you get out of bed and move a bit around in your home, you are already above BMR.

However, Garmin's resting calories are not pure BMR. They include a "base" level of activity on top of the pure BMR calories. So Garmin resting calories are approximately 20% higher than what you will see if you input your personal data in a BMR calculator. This also means that you will have to go above a certain activity threshold before Garmin starts counting active calories. Otherwise they would count the same activity twice. If I stay at home one day, doing nothing except moving around the house, my active calories in my Garmin watch will be almost 0, even though I of course have used energy on moving around.

So my guess is that Apple's resting calories are closer to pure BMR, which also means that Apple can/should use a lower activity threshold before they start counting active calories. But it is only a guess. I know absolutely nothing about Apple Watch.

That leads me to wonder what the activity threshold for active calories would be on the Fenix.

Would it be based off of heart rate zones?
What about active calories logged outside of an activity I.e. watch mode?
Or would it be based off of accelerometer data also? Does the activity class number or VO2 max calculation get taken into account?

Sorry, can't answer that. My feeling is that it is HR based rather than accelerometer based. If I move a lot when not in a logged activity, but I don't elevate my HR, I don't get many active calories.

I believe I can help a little bit here, as I too have been comparing the Apple Watch (Series 3 and Series 4) against Garmin Fenix 5x, and 5+ that I either have had, or now own. The Apple Watch's "Active Calories" are the calories that you have burned for that specific exercise, as noted either in an activity on the watch, or perhaps in the case of an app that writes to Apple Health, ie. WorkOutdoors, or others like that. The "Total Calories" are stated as the "total of active calories that you've burned, plus the inactive calorie burn for that period of time. So, it appears that the AW separates those calories that are actually burned during the exercise, from those that you would have burned watching TV, or some other sedate activity. I remember reading about this on one of the forums, probably the MacRumors forum. I too, have noticed that the total calories using either watch are pretty close, as are the charts, although the Garmin charts are worlds ahead of the Apple ones, IMHO.