https://arstechnica.com/gadgets/2019/01/facebook-and-google-offered-gift-cards-for-root-level-access-to-ios-users-data/

Both Facebook and Google have used Apple's Enterprise Developer Program—which is intended for exclusive use by companies to give system administrators the ability to distribute apps to employees' devices internally—to circumvent Apple's app store and distribute to users applications that closely monitor users' app, messaging, and network activity.