At the moment there is no end-to-end encryption as part of RCS. Google created a proprietary add-on for their app. Apple is working with the standards committee to add end-to-end encryption.

RCS on android has only two supported apps, one by Google and one by Samsung. Google is actively blocking phones with custom Roms from using their RCS app.

Google also provides/hosts the RCS backend software used by most telecoms. They can host their own implementation, and most did at the start. But now a lot switched to Google to provide their RCS service.

You can’t get around Big Tech spyware if you want to use RCS.