“In 2019, Mozilla called on Apple to increase user privacy by automatically resetting the Identifier for Advertisers (IDFA) on iPhones. The IDFA lets advertisers track the actions users take when they use apps – kind of like a salesperson that follows you from store to store while you shop, recording every item you look at. Creepy, right?
Early 2020, Apple went even further than what Mozilla supporters had asked for when it announced that it will give consumers the option to opt-out of tracking in each app, essentially turning off IDFA and giving millions of consumers more privacy online. Apple’s announcement also made a loud statement: mass data collection and invasive advertising don’t have to be the norm online.
Unfortunately, as you might imagine, a lot of advertisers, notably Facebook, were not happy with Apple. Facebook, which uses IDFA to track users’ activity across different apps and match them to advertising profiles, says that its advertising partners will be hit hard by this change.”

Source : Mozilla Foundation – Apple’s anti-tracking plans for iPhone