The camera position is based off where the mouse is on the screen, you can see by moving your mouse outside of the window then returning it at a different corner, the camera will move to that position.
There are many ways you could do this but i have replicated the behaviour using lerp to lerp the X and Y positions of the object that the camera is scrolling to, to the positions of the mouse in relation to the centrepoint but divided by 10 to limit the camera's movement.
Let me know if you have any questions :)
Demo here: 1drv.ms/u/s!AkmrWgxeuxlKhIcLU9fa3WTI5aMhMw