So I looked into this because my idea of RCS was that it was pretty decentralized direct encrypted communication.

TIL that RCS was designed as a fairly decentralized protocol where every carrier could run their own RCS servers, but it didnt get adoption. So Google stepped in and built their own routing infrastructure, and now mostly all RCS is centralized on Google's servers (even if you use an iPhone, because the RCS server is mandated by your carrier)