AR Foundation is Unity's high-level, cross platform API to support Augmented Reality. AR Foundation lets you write your app once, and build for either Android or iOS.
ARCore Extensions is a package that provides additional ARCore functionality for AR Foundation. ARCore Extensions can be downloaded and installed from the Unity Package Manager.
Cloud Anchors
ARCore Extensions supports ARCore's Cloud Anchors on Android and iOS devices.
For information on ARCore Extensions installation, environment configuration, and requirements, see the Quickstart for Android or Quickstart for iOS.
Try the Cloud Anchors codelab.