It's a heartrate wearable that focuses on really accurate heartrate variability measurement. This is important as it's an indicator of recovery.

This is the only article I could find that includes detailed information comparing the Whoops sensors to other devices: https://www.dcrainmaker.com/2020/05/whoop-3-platform-review....

Do you happen to know of others?

I don't have an article about it, but the reason i chose to replace my apple watch with a whoop was because i wanted insight into my HRV. Apple watch only measures HRV a few times a day, while whoop is continuous. I would only recommend a whoop if you are concerned you might be overtraining, that's what i use it for.

Looks like it is a fitness tracker of some sort.

https://www.whoop.com/

It’s a smartband or more like https://www.aidlab.com ?