How do you set up contacts for imessaging?
Imessaging: how to create the list, how to recognize an imessage from my list versus an outsider using imessaging and how to retrieve imessages, how to see who has sent the imessage
iPhone 13, iOS 15
How to create what list? Your phone will automatically recognize what contacts have iMessage and what ones don't.
You may also want to see this article:
Use Messages on your iPhone, iPad, or iPod touch - Apple Support