Once you set up your IDFA message on the Privacy & messaging page, your IOS ATT alert is automatically triggered when a user agrees to allow tracking on the IDFA explainer message. However, you must configure the NSUserTrackingUsageDescription on the iOS ATT alert by updating your Info.plist.
Learn more by visiting the developer documentation:
For AdMob: https://developers.google.com/admob/ios/privacy/idfa
For Ad Manager: https://developers.google.com/ad-manager/mobile-ads-sdk/ios/privacy/idfa