Sorry, I'm not sure what part of that wasn't clear. Facebook ships useful libraries to app developers as binaries and they use that to get code on everyone's phone in order to add to their data collection apparatus.
I'm pretty sure they're not the only ones who do that either. It's a pretty effective method to get around app sandboxes.
As someone who does not build mobile apps, none of that was clear from the earlier comment. I appreciate the additional explanation - I wonder if there's a way to find out which apps include this so I can avoid them...
But with the recent change to iOS 14.5 (and Android 12), those apps can't report to Facebook that they're being used by the same user (unless the user opts-in). Right?