RCS (Rich Communication Services) messaging is built into the stock Apple Messages app on iPhones and Google Messages on Android phones.
RCS messaging requires cellular data or Wi-Fi to work and offers more features than traditional SMS.
To check if your carrier supports RCS, go to IMS Status in your iPhone settings and look for the 'Voice, SMS & RCS' message.
To send an RCS message, open the Messages app, select a contact, and start typing the message. The typing field will show 'Text Message • RCS' if RCS is enabled.