Android

RCS is already enabled on iOS. It it doesn't work, that means your carrier or the carrier on the other end lacks support.

RCS is already made by an international standards body (except for the encryption part, that was invented by Google, which is why iOS doesn't do it). Your carrier is supposed to host RCS services on their network and your phone is supposed to register with that, whether it's a dumb phone or an app you downloaded like Google Messages. The problem with RCS is that carriers rarely implemented it when it came out a decade and a half ago, phone makers didn't bother to implement it, and the standard kind of bled dry.

The only reason it's gaining traction now is because Google said "fuck it, we're building an RCS server for Google Fi anyway, might as well let everyone connect" and so they set their RCS server up in a way the standard didn't foresee. This meant that everyone could suddenly use RCS, but the vast majority could only use it with Google's servers rather than their carriers'. They've even started renting out their RCS servers to other carriers because Google's server is a stellar shining example of great programming in the wasteland of awful telecoms service providers.

Apple isn't connecting to Google's servers and it's not running a telco network either, so their RCS works as long as your carrier bothered to set up RCS services, like they were supposed to do when they first rolled out 4G anyway.

Should be noted that standard RCS is unencrypted like SMS is, so don't use it to talk about things you wouldn't want the government to read back to you in ten or twenty years (crimes, future crimes like abortion, health information, maybe political opinions depending on how bad the government gets). Google added a layer of encryption on top, but Apple is refusing to implement it until it's standardised. And encryption won't be standardised, because the standards bodies work together with law enforcement. No point in adding encryption when carriers are legally banned from/directly liable for "damages" caused by rolling out encryption support. Hell, one of the major reasons Apple even supports RCS is because the Chinese government made them support it, and they sure won't accept encryption, they're having Apple run a special version of iMessage so the government can access everyone's messages for Peter's sake.

As for alternatives, there are a few ways for a phone to log into RCS, but rhe easiest way (a simple web request) works for any app on your phone. Others require SMS verification or access to the SIM card, which only system apps are allowed to do. None of this prevents phone manufacturers/custom ROM manufacturers from adding their own RCS API, or even other apps from registering themselves with an RCS server if your carrier doesn't require SIM access to do so. You can read the spec online if you want to know more about how it works, though it'll take you a couple thousand pages of reading if you want to know everything.

Maybe in time we'll see an alternative RCS client pop up. So far, nobody seems to be willing to pay for development or invest time in making an open source version, though.

They have rolled it out pretty extensively. My various conversations with Apple folks have all switched over.

Yeah, that's fair. I just meant they've built RCS, and kinda presented it as a replacement, but it's a walled garden they've built, unlike SMS.

I didn't know Apple might roll it out! That'd be cool! Though really what I want is just the possibility of an open source client