Flying Camera View Unity Project

Here is the code i used :
public static CameraController instance;

public float normalSpeed;
public float fastSpeed;
public float movementSpeed;
public float movementTime;
public Vector3 newPosition;

//CameraFollowObjectOnClick
public Transform followTransform;

//ZoomSettings
public Transform cameraTransform;
public Vector3 zoomAmount;
public Vector3 newZoom;

//mouseSettings
public Vector3 dragStartPosition;
public Vector3 dragCurrentPosition;
public Vector3 rotateStartPosition;
public Vector3 rotateCurrentPosition;


//RotationSettings
public float rotationAmount;
public Quaternion newRotation;


// Start is called before the first frame update
void Start()
{
instance = this;
newRotation = transform.rotation;
newPosition = transform.position;
newZoom = cameraTransform.localPosition;
}

// Update is called once per frame
void Update()
{
if(followTransform != null)
{
transform.position = followTransform.position;
}
else
{
HandleMouseInput();
HandleMovementInput();
}

if(Input.GetKeyDown(KeyCode.Escape))
{
followTransform = null;
}

}

void HandleMouseInput()
{
//ZoomMouse
if(Input.mouseScrollDelta.y !=0)
{
newZoom += Input.mouseScrollDelta.y * zoomAmount;
}
//MoveMouse
if(Input.GetMouseButtonDown(0))
{
Plane plane = new Plane(Vector3.up, Vector3.zero);

Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

float entry;

if(plane.Raycast(ray, out entry))
{
dragStartPosition = ray.GetPoint(entry);
}
}
if (Input.GetMouseButton(0))
{
Plane plane = new Plane(Vector3.up, Vector3.zero);

Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

float entry;

if (plane.Raycast(ray, out entry))
{
dragCurrentPosition = ray.GetPoint(entry);

newPosition = transform.position + dragStartPosition - dragCurrentPosition;
}
}
//RotationMouse
if(Input.GetMouseButtonDown(2))
{
rotateStartPosition = Input.mousePosition;
}
if (Input.GetMouseButton(2))
{
rotateCurrentPosition = Input.mousePosition;

Vector3 difference = rotateStartPosition - rotateCurrentPosition;

rotateStartPosition = rotateCurrentPosition;

newRotation *= Quaternion.Euler(Vector3.up * (-difference.x / 5f));
}
}

void HandleMovementInput()
{
if(Input.GetKey(KeyCode.M))
{
movementSpeed = fastSpeed;
}
else
{
movementSpeed = normalSpeed;
}
//GoUp
if(Input.GetKey(KeyCode.Z) || Input.GetKey(KeyCode.UpArrow))
{
newPosition += (transform.forward * movementSpeed);
}
//GoDown
if (Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.DownArrow))
{
newPosition += (transform.forward * -movementSpeed);
}
//GoLeft
if (Input.GetKey(KeyCode.Q) || Input.GetKey(KeyCode.LeftArrow))
{
newPosition += (transform.right * -movementSpeed);
}
//GoRight
if (Input.GetKey(KeyCode.D) || Input.GetKey(KeyCode.RightArrow))
{
newPosition += (transform.right * movementSpeed);
}

//RotationKey
if (Input.GetKey(KeyCode.E))
{
newRotation *= Quaternion.Euler(Vector3.up * rotationAmount);
}
if (Input.GetKey(KeyCode.A))
{
newRotation *= Quaternion.Euler(Vector3.up * -rotationAmount);
}
//ZoomKey
if (Input.GetKey(KeyCode.R))
{
newZoom += zoomAmount;
}
if (Input.GetKey(KeyCode.F))
{
newZoom -= zoomAmount;
}

//GetSmouthMovement
transform.position = Vector3.Lerp(transform.position, newPosition, Time.deltaTime * movementTime);
transform.rotation = Quaternion.Lerp(transform.rotation, newRotation, Time.deltaTime * movementTime);
cameraTransform.localPosition = Vector3.Lerp(cameraTransform.localPosition, newZoom, Time.deltaTime * movementTime);

}