I have an iPhone with The Weather Channel app. During stormy weather the app will pop up a message telling me that lightning has struck so many miles from my current location. I know where it’s getting my location data, but how does it know that lightning struck in some random place? It does not appear to be using a sensing device installed locally on my actual phone.
Your app (my AccuWeather app gives the same sort of alerts) is undoubtedly tapping into information from one or both of the lightning detection networks described on this web page.
If one watches a weather broadcast (such as on a local TV station, or The Weather Channel) during a severe thunderstorm outbreak, they will often display locations for lightning strikes as part of a radar image of the storms. That data comes from the same source.
That same source is also used by public facilities (youth sports and pools in particular) to warn people when they need to seek shelter.
ETA: Scout camps also use them for lightning calls to get people out of the lake or pool.
At home we use a dedicated lightning app (Lightning Alarm) to warn us when we need to get out of the pool and inside. Monsoon storms can form and move in really fast.
It is not necessarily coming from your phone. The data is triangulated using electromagnetic signatures and timing information; there are kits you can buy:
Phones could even be the original source for the information. There are lots and lots of antennas in a phone, they can all talk to each other over the Internet, and they’re spread very widely, concentrated in the places people care most about.
Weather Underground also uses a network of Personal Weather Stations, and it wouldn’t surprise me if they share their data. I have a separate lightning detector for mine, but that sensor doesn’t share data. Some weather stations have lightning detection built in so it does get shared.
Quite possibly, especially since the same company (The Weather Company) which owns Weather Underground also owns the Weather Channel mobile app (but not the Weather Channel’s TV service, which is owned by Byron Allen’s Allen Media Group).
The Weather Channel owned Weather Underground from 2012 to 2015; when it was spun off to IBM in 2015, that spin-off included the Weather Channel app.
The maps from those sources are fascinating to watch:
(The latter map is shinier and has more bells & whistles but is also covered with ads. Choose your poison.)
I doubt that would work for anything but a very close lightning strike. Most of the energy is in the low frequency range, for which smartphones don’t have antennas (and are too small to have useful ones).
At the frequencies for which there are antennas, I doubt there is any way to retrieve the raw received waveform in order to analyze it.
All you could maybe do is get information on signal to noise ratio for multiple frequencies, and if noise spikes on all of them at the same time, you could assume a lightning strike was responsible.
But every lightning strike is a very close lightning strike. Cell phones are everywhere.