arfoundation-samples/Assets/Scenes/FaceTracking/ToggleCameraFacingDirection.cs

using UnityEngine;
using UnityEngine.XR.ARFoundation;

public class ToggleCameraFacingDirection : MonoBehaviour
{
    [SerializeField]
    ARCameraManager m_CameraManager;

    public ARCameraManager cameraManager
    {
        get => m_CameraManager;
        set => m_CameraManager = value;
    }

    [SerializeField]
    ARSession m_Session;

    public ARSession session
    {
        get => m_Session;
        set => m_Session = value;
    }

    void Update()
    {
        if (m_CameraManager == null || m_Session == null)
            return;

        if (Input.touchCount > 0 && Input.touches[0].phase == TouchPhase.Began)
        {
            if (m_CameraManager.currentFacingDirection == CameraFacingDirection.User)
            {
                m_CameraManager.requestedFacingDirection = CameraFacingDirection.World;
                m_Session.requestedTrackingMode = TrackingMode.PositionAndRotation;
            }
            else
            {
                m_CameraManager.requestedFacingDirection = CameraFacingDirection.User;
                m_Session.requestedTrackingMode = TrackingMode.RotationOnly;
            }
        }
    }
}