Most rectangular pools have straight cut copping at the corners, the round look is the rare exception done by request for personal taste. The standard straight corner has the stone cut at a 45 degree angle, as seen in the attached. photo.

I am assuming there is some type of white top mount copping underneath the stone for the liner? Was that rounded in the corners or is it straight? Does it matter for the straight corner look? I was under the impression the cantilever coping follows that track so if the white top track is rounded in the corners so would the cantilever coping. What should be done to the track underneath the stone in each corner of the rectangle to achieve the straight look in your attached picture - thanks

Apologies for delay in responding. The channel to secure the liner usually runs right into the corner and is fairly square. It sometimes requires a skilled installer to insert the top bead of the liner into the channel in the corners. I believe experienced installers each have their own techniques for handling this aspect of the installation.