>They provide third party API's to use APPLE's RCS-Service. The alternative would have been to support registering alternative RCS-services as default on the OS (and then, allow the user to choose a service).

RCS messaging is carrier-controlled and configured via carrier bundles in iOS. Apple doesn't run a "RCS service". TelephonyMessageKit [0] in iOS 26+ exposes a standard interface to the carrier SMS, MMS, and RCS services, as applicable, allowing for 3rd party applications to send and receive carrier standards-based messages.

In 3GPP standards, RCS is just another service using the IP Multimedia Subsystem (IMS) framework. Carriers can either run their own RCS service in their IMS core or use a 3rd party service (as many do with Google's Jibe).

[0]: https://developer.apple.com/documentation/telephonymessaging...