It comes from aircraft equipped with ADS-B transponders. Not all aircraft have them, Airbus always has, Boeing, I think does now on later versions, some private aircraft have them but it’s not mandatory. It will not show aircraft not equipped or aircraft flying in areas not covered by an ADS groundstation. If you want to look at the aggregated result install Flightradar24 on your device and watch all the world’s aircraft flying anywhere. If you live in a remote area not covered by a groundstation you can fill that area by setting up a station and connecting it to your internet. Probably, you will be able to get a receiver for free if your area is not covered.