Origin of pings

I wanted to know a bit more about the origin of the pings that Safegraph uses. My understanding is that it aggregates data from many apps that record these location pings. Does Safegraph only use data from apps? Could you give some examples of apps which Safegraph gathers data from? How many apps in total does Safegraph get data from? Is there reason to believe that these sources oversample specific population segments?
This is related to another question: does Safegraph also have data on visitors from foreign countries (apart from Canada)? Should we expect lower coverage for these visitors?
Thank you very much!

@Niki_from_SafeGraph, do you have any tips on this by any chance?