Google Messages (and I think Samsung Messages) is the only app that supports RCS.

I'm interested in knowing too!

Google messages also has some annoying moments where the RCS will just quit working for no reason at all, first falling back from e2ee to standard RCS which is very annoying, and in some cases it will drop even further back to SMS if you haven't prevented it from doing so in the settings.

The messages app feels great to use, but it's privacy attempts aside from the e2ee are just not there IMO