Send notification when app is force closed

Apps that require you to keep it running in the background like Whoop, Tesla etc send a notification when you force close it.

How do they do this ?

As far as I know there is no way to detect reliably when you app is about to be killed.

appWillTerminate does not trigger if the app is already in the background

Ive tested the Whoop app by turning off bluetooth and no internet connection and I still get the notification. So its not relying on remote/backend updates.

Any ideas would be greatly appreciated