If you miss your flight (on same ticket, even if CA marketed) because of AY's own delay or the AY operated PEK flight is delayed or cancelled, AY is responsible to provide the rerouting and duty of care.

I.e. no difference in case of irrops when it comes to codeshares, also the same with AA/AY/BA, the carrier "causing" the delay should take care of the customer. It is a different story, if one wants to change the ticket before commencing travel, when one needs to be in touch with the issuing carrier.