Samsung has chosen to drop its own messaging app, Samsung Messages, starting with the Z Fold6 and Flip6. Instead, Google's messaging app is the default.
Samsung drops Samsung Messages on new mobiles
The reason is that Google is now betting hard on RCS, especially now that the iPhone will be supported starting with iOS 18 in September (it already works in the beta versions.) Samsung reports that even if its own SMS app also supports RCS, it will be rolled updates and support faster and more widely with Google's app - not least it is much safer to use Google's app, because then you know that the person you are communicating with will receive the messages and that with full RCS compatibility:
“Although messaging apps follow the RCS standard, availability may be limited depending on which app the other party is using. That's why we decided to make Google Messages the default messaging platform, so Galaxy users can communicate more freely. This also enables a messaging app to respond to changes in the RCS standard more quickly and efficiently.”