Seemingly every device does things slightly different. I don’t know what metric garmin uses (actually it would be First Beat who does the software for that feature) but they take some metric and then translates that into the “Stress score” you get on the device.
rMSSD is what HRV4Training uses and they translate that to their “recovery points” on a 1-10 scale. I know that Apple uses sDNN as the metric they record as HRV… and the guy behind the HRV4Training app has verified that they can translate that into their system for recovery points. There are some other measurements that could fall into the HRV category but those are the main two that I’ve seen.
I can’t speak to whoop or any of the other guys out there. I just haven’t taken the time to look into it because I’m happy with the app I use and it works well for me. They also have some good academics behind their software (not that the other don’t… just that I haven’t read anything on it)
The main thing is to pick one system and stick to it. Like I said before, it’s about knowing YOUR baseline and watching if you vary too far from that.