Why Mac and iPhone sent message are separated?
You can send messages from different phone numbers and email addresses linked to your Apple ID. You can check those under:


(on iOS) Settings.app -> Messages -> Send &amp; Receive
(on macOS) Messages.app -> "⌘," (Preferences) -> iMessage tab


If you are sending from different addresses and the receiver does not have both addresses saved in the same contact they will definitely show up as separate conversations.