This method allows you to apply added GameObjects to an existing Prefab. It mirrors the Apply functionality in the Editor, as described in the Apply overrides to the prefab asset documentation. To use this method, you must first add the GameObjects to an existing Prefab instance.
An added GameObject is a type of Instance Override. The act of applying added GameObjects to the Prefab means they will become part of the Prefab Asset, and will no longer be overrides on the Prefab instance.
When applying added GameObjects to a Prefab Asset, you must supply the asset path as a parameter. This is because there are some situations where there are multiple possible targets to apply the change to. For example, if added GameObjects have been added to a GameObject that is part of a nested Prefab, you may have the choice of applying the changes to the inner nested Prefab Asset, or to the outer root Prefab Asset. Therefore, by specifying the asset path, you make it clear to Unity which Prefab Asset the change must be applied to.
For more information, refer to Override prefab instances.
Additional resources: PrefabUtility.ApplyAddedGameObject, PrefabUtility.ApplyAddedComponent, PrefabUtility.ApplyObjectOverride, PrefabUtility.ApplyPropertyOverride, PrefabUtility.ApplyRemovedComponent, PrefabUtility.ApplyPrefabInstance.