How will Apple iPhones exchange RCS messaging with Android phones?

iMessage will continue to be supported by Apple between iPhones so RCS will only be used for messaging between iPhones and Android devices with Google Messages. How will Apple recognize when a Google subscriber has received an RCS message (and vice versa). Will an Apple user be able to see when a Google user is typing (and vice versa)? Will there be peer-to-peer RCS messaging between originating and terminating devices? Is there any onus on the mobile network carrier (some large, some small) to ensure that RCS messages flow both ways correctly?