RCS is a messaging service / protocol developed by the GSMA (the global system for mobile communications), a non-profit industry association representing mobile telephony operators worldwide. Nothing to do with American phone companies.
For someone who is "not even sure what RCS is," you seem very certain of that.
Per Wikipedia:
"In early 2020, it was estimated that RCS was available from 88 operators in 59 countries with approximately 390 million users per month. By November 2020, RCS was available globally in Google Messages on Android, provided directly by Google if the operator does not provide RCS. By 2023, there were 800 million active RCS users on Google's platform and 1.2 billion handsets worldwide supporting RCS."
