That's fine. Allow me to temporarily remove Apple here to try and make it clearer. Samsung recently joined up with Google for RCS. Prior, Samsung always used "Samsung XYZ" apps on their devices for everything, Samsung Messages in this specific case. That's what they shipped Galaxy devices with. Samsung Messages also had its own RCS platform, but it could not send an RCS to Google Messages, nor AT&T Advanced Messages, or Verizon Advanced Messages, etc. Google worked hard to get Samsung on board, and they scored them -- with the S21 in some markets Samsung Messages was no more, it shipped with Google Messages. The S22, the US got it and basically all other markets. Why didn't they just hook up Samsung Messages? Seems like an easy solution, Samsung already had a lot invested in their own app development. But it's because there's no avenue in - the API that they don't have available. Google has it closed off.I'll happily admit you're talking over my head with the first part. I'm still not sure what the point would be for Google to release Messages as RCS only, though. Too many iPhone users (at least in the United States) rely solely on iMessage. So you still get a lot of angry (android) customers who would have no way of messaging their friends/family on iPhones. It would be the same situation as trying to get people onto WhatsApp, which from my own personal experience, has not gone well.
Now, for Apple to join on board-- They'd want it within their own Messages app alongside iMessages and have it be the step-down, and sit above SMS. Same problem here, there's no way in to Google RCS from anyone not named Google. Google wants iOS though, but it wants the SMS fallback. On the other side of the coin Apple won't give anyone, not just specifically Google, access to the core OS to get to its SMS functions. That's why there's no alternate texting app on an iPhone. The OS itself is closed source, unlike Android where the core OS, the AOSP part, is open source - and then Google throws a bunch of closed source apps on top and "Android" - what we buy as consumers - is what we get.