It literally is Google-only. The RCS backend theoretically could be provided by carriers, but they've all chosen not to do that, so the actual service is provided by Google. No matter what the specification says, in reality it's a Google service running on Google's servers.
To put it another way, Google can't kill SMS short of literally removing the app from Android because it's not their infrastructure, but if they shut off their RCS servers tomorrow, it would be dead for good. That's a Google-only service.
It's sad to see so many people are blinded by this. The current situation of RCS is just that Google saw Apple disguised iMessage as SMS and wanted to do the same. RCS is merely a vehicle for Google.
They could just layer their own chat platform on top of Google Messages but we all saw how Google's IM business went along: Chat, Hangouts, Alo, Meet etc. So they muddied the water so deep (to a carrier level) to make it look like it's Apple's issue for not adopting RCS. And people actually fall for it.
Nobody wanted RCS. Even carriers don't want to maintain RCS. They just use Jibe. And that's exactly what Google wanted. My RCS communication with friends don't even show up in carrier's usage. How is that ever different from iMessage...
You know who chose to selfhost their own RCS server? Yes, Chinese carriers! They call it 5G Message. New ad delivery channel for businesses hooray! Instead of plain text and a link, now your campaigns can even have MENUs inside! I can send SMS to a Chinese number, I can send iMessage to a Chinese number, but I can't send RCS. Truly "Universal" profile.
I agree with all of this except for the claim that "Google wanted this". I think Google is as annoyed with this situation as everyone else. They would've preferred to have their own iMessage alternative, but they launched a dozen which all failed, so they went "Well, we can't make our own that people want to use, so let's get the carriers to make an upgraded version of SMS". And then the carriers didn't want to do that but the "it's decentralized!" message stuck with users and even a few governments, so now RCS is the worst of all worlds: it's a de facto Google service, but with a janky, half-baked decentralized protocol, where Google has limited capability to improve it compared to a native Google chat app.
Right, and does that use Apple's servers? This is a rhetorical question, we both know it goes through Google, both literally and figuratively. Google effectively controls RCS - if Apple just implemented RCS 'per spec', it would not work. So, there is no spec, it's as if it doesn't exist. That's how that works.
And there is one singular app which supports RCS.
In many ways, it's a regression from SMS. In that SMS is somewhat universal, and RCS is so specialized it's almost worthless.