I will setup facebook conversion API with GTM fb pixel ga4 ecommerce tracking

Why need Facebook conversion API? Due to the iOS 14 update and Safari, Firefox, and Mozilla have intelligent tracking prevention (ITP) algorithms. FB pixel only track browser-side events which are giving incorrect event tracking data. FB can't track accurate data for iOS users. To fix the issue, you need to capture data directly from the server instead of browser-side through conversion API or server-side tracking.