iOS did boycott RCS for a long time in order to convince US teens to buy iPhones rather than Android phones (due to incompatibilities between iMessage and SMS, which are popular in the US). It worked. I think there was even some leaked internal email which admitted to doing this. The US antitrust agencies ignored it; there were no legal consequences for Apple. I believe Apple finally decided to add RCS support after EU legislators threatened to force their hand.

About a billion people use it every day, but they're concentrated in specific countries. I believe India is a major RCS user for instance.

Like Signal, it works fine, but you need your contacts to also use it. That's easy on Android (Google hosts an RCS server for people whose carriers don't) but carrier dependent on iOS.

RCS video calls and payments seem to be left unimplemented by Google.

iOS and Android both use RCS

iOS as a fallback messaging system since iOS 18, before falling back to SMS

exposing which people are google voice numbers vs actual android devices