Anchors

place wg at (0, 0) moved the center of the rectangle to the origin. Every shape has an origin — for a rectangle, its center — and place uses it unless told otherwise.

Every shape also has anchors on its bounding box:

top_left       top_center       top_right
left_center      center      right_center
bottom_left   bottom_center   bottom_right

Name an anchor after a dot, and place moves the shape so that this anchor lands on the target:

place wg.left_center at (0, 0)     # the waveguide starts at the origin
place pad.bottom_left at (0, 0)    # the corner of the pad is at the origin

wg.origin names the origin itself. A shape also knows its size: wg.width and wg.height are the width and the height of its bounding box.