AFAIK the local optician orders prescription lenses that come in standard, oversize (probably circular) profiles.
The frame manufacturer publishes a shape code which is plugged into the optician's local shape-grinder machine to make the lens fit the frame. Or a drawing, in which case the optician uses his manual skill to cut the lenses to suit. (Perhaps that's why some opticians can't make lenses to fit some frames).
At least that's how my guy does it this side of the pond – I can't believe it's any different over there.