In theory, RCS is something cell phone carriers should support directly, like SMS and MMS. In practice, carriers just didn't care, so Google built their own RCS relay ~~with blackjack! and hookers!~~, integrating it into their own Messages app. This works because RCS is designed to run over the internet, and does not strictly need a phone network. Unfortunately, it means you need to use Google's Messages app.

While Android allows third-party developers to hook into SMS and make their own texting apps, Google has not built RCS support into Android itself — it's an application-level feature. Currently third-party RCS clients are not possible. I think Google has partnered with Samsung (and maybe others?) to include support in their messaging apps as well, but fans of third-party apps like Textra are out of luck.

I'm not totally sure what the situation is on carriers who directly support RCS. Some of them have simply partnered with Google rather than building their own support. Some of them only support it within their own network (an early-2000s throwback nobody wanted). It's been a mess and I'm not up on all the latest developments myself.