How to Enable RCS Messaging in iOS 18
Apple introduced RCS support with iOS 18. However, full functionality will depend on carrier support. The good news is that major U.S. carriers like Verizon, T-Mobile, and AT&T already support RCS. Apple also has a list of carriers that support RCS on its website.

Open Settings on your iPhone and select Apps at the bottom of the menu.
Tap Messages.
Under "Text Messaging," select RCS Messaging.
Toggle RCS Messaging to the green ON position if it isn't already enabled.

If you don't see the setting on your iPhone, that's likely because your carrier does not yet support it. Check Apple's wireless carrier support page to check if yours is listed.
How to Tell If You're Using RCS

There's a simple way to tell that you are sending an RCS message in the Messages app. You'll see "RCS" at the top of the conversation window as soon as you type in the number of someone not using an Apple device.
The Future of RCS

Apple isn't stopping at just implementing RCS. The company plans to work with Google and other members of the Global System for Mobile Communications Association (GSMA) to further improve the RCS standard worldwide. This collaboration should lead to even more advanced features, such as the ability to edit and delete sent RCS messages.