@dbennett0100
There has been already discussions about the question of this density:
Is it per hour or per day?
https://community.windy.com/topic/3361/description-of-weather-overlays/28

The thunderstorm layer is expressed in lightning per square kilometre. It is the average density of likely lightning strikes computed on a wide area but then expressed per square km. So it is not within a radius to your location, just a potential density.

To clarify these 2 questions, see more detail from ECMWF about this parameter:
https://apps.ecmwf.int/codes/grib/param-db/?id=228057

Finally I understand that ECMWF compute the potential of lightning strikes density during 24hours but they are displayed at a precise hourly time frame. It is about the same when you are driving, your speed is expressed in km per hour (or miles per hour), but your speed may vary by each minutes.

@idefix37

On the app... quite possibly depends on the type of phone & OS version + available phone memory & battery saving settings.

My Samsung with Android 7 couldn't do this. It could only remember the last opened layer + forecast model for a short while, maybe a few minutes after closing the app. But after a longer time, it would always open on Wind + ECMWF (which is the default settings on Windy)... However, after upgrading the OS to Android 8 Oreo, my phone seems to always remember the last opened layer + forecast model.

EDIT: After some very long while (didn't get the chance to measure the exact value; let's say just after the night as I am opening the app so often during the day :)), my Android app does open on Wind + ECMWF.

Forgot to mention that I'm a beta-tester (version 16.12.3 at the moment)... But my friend's regular Android app always opens on Wind + ECMWF by default.