Comment by izacus

I think it'll get clearer when you realise that SMS (and MMS) is also nothing more than a "data connection" message that carriers chose to classify as zero-cost and available during roaming via different pricing.

RCS - and VoLTE for example - is in this way no different. It's just a data message that gets sent to a configured endpoint (either on Google or your carriers' servers, depending on your carrier preference).

Of course, the carriers still remain the dumbass link in this chain as usual. I've just had to deal with a carrier that charges VoLTE call configuration setup connection as normal foreign data roaming with minimum pricing. The phone ate through 30EUR of costs despite having mobile data disabled.