Re: how does "code share" work?
Codesharing just allows an airline to extend its network by getting another airline to fly stages on its behalf. If you look in the BA timetable, you will see that BA puts its code on a flight between HKG and Auckland which is flown by CX. BA puts its code on several CX flights around Asia - cities like Kuala Lumpur. It some cases this does not always give you the shortest time between London and those cities - why would a BA passenger want to fly to HKG and then fly three hours back to KUL? BA has done the deal with CX and the passenger has a longer journey time.
The financial arrangement is that BA would get a cut of the money that CX gets for flying the HKG -AKL leg.
You can have more than one airline code sharing on a service. If you look at the departure boards at the airport, you will see three airlines listed for what is the one flight.
Someone should offer a prize for the most interesting round the world itinerary where you do not ride on the airline you thought you were going to be on