For an AA codeshare number on a CX operated flight, CX gets the revenue because it operates the flight. Then, because it was sold under an AA flight number, CX pays AA a referral fee for the sale. This is one of the reasons that 1) there are relatively few AA codeshare numbers on CX flights, and 2) the AA codeshare inventory on any particular flight is usually lower, often much lower, than the CX inventory for the same flight (check this out on EF), -- CX doesn't want to pay and would rather try to sell the seats itself.