Google does control the proprietary extensions to RCS they use to try to catch up to iMessage on security and features. The developers of apps like Signal, etc. have been asking for access for many years but Google chose to exclude them as they try to build their user base. Similarly, most of the carriers in the US haven’t actually implemented it themselves - they’re just paying Google’s Jibe subsidiary to host it for them. This is not open in practice even if there’s a theory where it could eventually be open.