As far as I know Google doesn’t allow third party apps to plug into RCS.
This is why them bashing Apple for this particular issue always seemed hypocritical to me, they want this to be their own closed ecosystem, with Apple being the exception because they have enough clout to actually go it alone or even take users away from Android.
Ideally you’d have apps like Signal plugging into the same end-to-end encryption for interoperability, but Google won’t allow that because they just want people to use Google Messages for RCS, and nothing else.