Originally Posted by
nerd
I think the selling carrier buys the seat from the operating carrier, and re-sells it to pax for whatever it wants to charge.
Please see immediately preceding post, which beat me to the punch and is quite accurate. That is one of many options.
What it comes down to is that the two airlines can make any kind of agreement that makes sense to them. There are no rules.
Frequent flyer program mileage accrual is unrelated to how the code-share contract between the airlines works. In oneWorld, credit is determined by who sold the ticket. In Star Alliance, it's based on who operates the flight. So, if you buy a seat on a flight operated by Airline A as Flight XXXX on Airline B, in *A you get credit as if it's on A, but in OW as if it's on B. (Don't know about other alliances.) This matters when the mileage earned for flights on A and B in a given fare class is different. (A complication in *A is that a ticket bought from B in a given fare class may translate into a different fare class on A, but it can be hard to find out what that is because the passenger only sees what the selling airline, here B, sold.)