The carriers (full members of GSMA) came up with RCS as a replacement for SMS/MMS at a network level and then didn't have the appetite to implement it.
Instead, they either A) adopted Google’s Messages as is, B) adopted Google’s Messages in white label form, slapping their logo on it, or C) made “their own thing” running on Google’s Jibe servers.
With very few exceptions, it all goes through Google. And Google uses this to pretend that their proprietary iMessage competitor is “the” RCS standard.
The actual RCS standard is dead for all intents and purposes because nobody uses it like that, except for Apple in the short term.