Those using iPhones cannot access RCS. Those using Android can use RCS if they both use an RCS-capable messaging app and have signed in to RCS messaging.
There are currently only 2 RCS-capable messaging apps, Google's Messages app and Samsung's app (which has the same name), so anyone who uses a different app will not have access to RCS. I do not know why Google will not allow anyone else to use the APIs: it's like they want everyone to use it, but also want to be Apple-like in their control. Maybe there's a good reason, maybe there's a bad reason, maybe there's no reason at all - Google have never explained.
As for signing in, I suspect many people don't even know it exists: lots of people just use the default app with the default options and never look at the settings, in which case they'll never realise it's there. And not all of those who do know will bother: I don't know anyone who uses it, so for me it's not worth doing, especially since that would mean using either Google's or Samsung's app, both of which I find sadly lacking in customisation compared to what I use myself (edit: I'm another Textra user).
Personally I find it easier to use WhatsApp or Signal: work with both Android and iOS, and while Signal is more select everyone I know has WhatsApp so it's essentially replaced SMS (I get SMS from companies, but personal SMS have been rare for some years now).