"Report Junk" was originally added for iMessage (with reports going to Apple, I assume). iOS 16 expanded the feature to support reporting SMS/MMS "Depending on your carrier and country or region": https://support.apple.com/guide/iphone/block-filter-and-repo... (scroll to "Report spam or junk messages").
"In the list of messages, touch and hold the spam message, then tap Report Junk. Or, if you’ve opened the message, scroll to the bottom of the message, tap Report Junk, then tap Delete and Report Junk."
I wonder if that's what the iPhone's "Report Junk" button does with text messages.
For some reasons, my iPhones on AT&T always offer the option to report a text message as spam, but my iPhones on Verizon do not. Another curiosity.