Might surprise you, but with RCS you have to have an account - but one that your phone operator manages. You have to use a server - just one that your operator choose and usually they lent them from Google. And you have to use an app - just one baked in the modem that you cannot update or change yourself.
There is no peer-to-peer in RCS like with SMS, there is a remote account you create and connect to when launching Google Messages app. It’s just a chat app with additional steps.