RCS dates back to 2007/2008 when it was still called lots of other names. (E.g. Joyn) And since then, not many cell providers adopted it. For all other providers (and those still sitting on an old version of RCS), communication will happen via Google-servers. It basically is a proprietary service under the disguise of a public standard. Especially because of this I’d rather use “proprietary” encrypted chats with it, so Google doesn’t get a copy of all my texts.
No need for reverse engineering - it has already been done: https://github.com/RhetTbull/osxphotos