There is no such thing as a cost breakdown by segment. You likely have one ticket with one price, whose routing rules allow you take the segments you are taking.

I'm not sure what you mean by an AA codeshare on AA, but if some of your flights have AA flight numbers, they will likely prorate the total fare by distance (though they are not required to; they can do it however they want. There it no "right" answer).

This is a better question for the AA forum, since it's about how AA's program works.