They paid users to install an app on smartphones that spied on network requests made by those users. In iPhone this app used OS VPN support to intercept traffic, and a root CA certificate to treat the intercepted connection as legitimate.
“Proxyman” is an iOS app that does the same thing on behalf of the user - it’s super helpful for debugging your own app or reverse engineering someone else’s app. I used it to reverse engineer the API of my smart home gym app Tempo Fitness so I could build my own dashboards from my workout metrics.
Facebook used this technique to “wiretap” the analytics log events that Snap and the other apps were sending to themselves - events probably look like “user swiped to next video after viewing video id=1234 for 3 seconds” or “user clicked ad id=5678”.
TechCrunch report on Facebook Research (2019): https://techcrunch.com/2019/01/29/facebook-project-atlas/?gu...
The Wikipedia page for Onavo (the startup Facebook bought that that powered this stuff) is pretty clear and has good citations for additional reading: