in the messenger app, one can create a message or post
I still could not figure out the difference
can someone enlighten me?
This is addressed (sort of) in this KB article:
As far as I can tell (which is not very far), this applies to situations where there are multiple cell number recipients. In particular, how responses are handled (and threaded). A "New Message" is like a group text. Everybody sees the original message and all responses from other participants. A "New Post" is (I guess) like sending the same message to all the recipients. But responses are visible only to the original sender and the responder. That is, the responses are threaded individually.
All of this clearly has a use. But group messages and posts seem excessively complicated. IMO.