Every RCS message sent via Google Messages apps in all of the countries outside a few countries (China, NK, Russia, Pakistan, Bangladesh, some telcos in Germany) is (or at least WAS) sent to Google Jibe RCS message servers, AFAIK. This situation may be changing now, as some carriers are now starting in some countries to run the RCS servers (perhaps in a Google cloud?) themselves. More info in the following thread : https://discuss.grapheneos.org/d/1353-using-rcs-with-google-messages-on-grapheneos/1441
At the very minimum, Google stores metadata about the messages: phone numbers of sender/receiver, IMEI device and SIM card identifiers. This is the type of metadata that is normally stored on your telecom carriers servers, if you plain old SMS/MMS messaging instead of RCS. Usually when you send only SMS/MMS the message portion itself is also stored on your telecom operators servers (the storage time depends on telecom provider/country).
With Google RCS messages the message content itself (text, videos, photos, sounds, other binary blobs) are encrypted on device, sent to Google servers encrypted, downloaded onto receiving devices, and under normal circumstances deleted from Google servers within 24-72 hours (UCB security researchers found that messages are deleted usually within 48 hours in "most cases").
AFAIK (far from being an expert), the message content itself is encrypted with AES-256-CBC (similar to what Signal.app at least used to use, have not checked if they've changed it). This is considered for now to be very secure encryption algorithm in terms of being fairly resistant to brute force attacks.
As for the list of specific identifiers/data that Sandboxed Google Play services omit (i.e. don't send) when using Google Messages app with Google Jibe servers during RCS message transmission, I don't know. I hope somebody who has actually looked at this in detail will answer that, as I'm also interested in finding out. Different sources give different information about this.