UnityBasicCameraExample.cs

void Update () {

    // ...
    
    // - Body
    //   \_ - Camera

    // Rotate full body right and left
    transform.Rotate(
        0,
        Input.GetAxis("Mouse X"),
        0
    );

    // Rotate camera attached to body up and down
    camera.transform.Rotate(
        Input.GetAxis("Mouse Y") * -1,
        0,
        0
    );

    // ...
}