Does anyone know exactly how codesharing works on a technical level? With airlines using a wide variety of back-end GDS systems like SABRE and Amadeus, it must be technologically difficult to link the two systems to open up codeshares on 2 airlines. Does anyone know if codeshares are done via a live link between two systems? Or are codeshares bought from the inventory of the airline you are purchasing from?

In other words, if I'm on AA.com and I buy a 6xxx flight that's codeshared on BA, does that inventory for BA's flight come from AA's SABRE or does it come directly from BA's Amadeus?

I would assume the latter since buying flights on BA from AA.com without a codeshare is also possible.