Best flexibility: Bluetooth chest straps like the Polar H10 work with almost every third-party app. If you want freedom to switch apps, start here.
Locked ecosystems: Whoop and Oura devices only work with their own apps. You get a polished experience but can't use alternative software.
Apple Watch: Limited third-party HRV app support. Some apps use the phone's camera instead of the watch sensors.
Camera-based: HRV4Training and Welltory can measure HRV using your phone's camera—no hardware needed, though accuracy is lower than dedicated sensors.