There's not really much of a point using the RCS Test app, as it's only a demo of very basic RCS features.

The thing with RCS is that whilst the "official" spec (ie the Universal Profile), as defined by the GSM Alliance, is open, it doesn't implement or define many modern of the chat features found in modern apps, such as reactions, replies, end-to-end encryption etc. These features however, have been implemented by Google in their Messages app and their Jibe backed service. The problem is that these additions by Google are proprietary and only works via Google's Messages app, so third-party messaging apps can't get in on the fun.

I believe Samsung's Messages app may also have access to some(?) of these features if the cellular carrier also uses Google's Jibe servers for RCS routing, but don't quote me on that.

As for Apple, I'm pretty sure that if they implement RCS (supposedly this year), it'll either be the Universal Profile, or most likely the Universal Profile + some proprietary Apple magic sauce for added features. Not sure about E2E encryption though - they would have to work with Google for that to work (for interoperability with Messages), so we'll have to see how that goes. If I were to guess, I'd say E2E on Apple would most likely be limited to Apple devices. But at least we can expect basic rich messaging features to work cross-platform, so that's something I guess.

In any case, the main issue remains that Google hasn't opened up the API/spec for their version of RCS - and the GSMA is seemingly doing nothing about it either, the Universal Profile hasn't had any updates in the last four years. You can read about the spec in detail here, and if you do, you'll see that there's no mention of modern chat features such as end-to-end encryption...

So on one hand, it's a good thing that Apple is getting RCS this year, but it'll likely remain either the at the basic Universal Profile level, or some proprietary Apple stuff thrown in, both of which aren't really ideal.

For the rest of us, none of this really matters unless Google opens up the spec, because why the heck would you settle for a somewhat insecure and limited protocol, when there are far better messaging apps out there, with a greater userbase and cross-platform interoperability?