Hi there thanks for posting in the community! To comment on a specific object simply select it and then choose the Comment button in the Context Toolbar. 

This will open the comment panel where you can type your comment. To direct it at a specific collaborator you can mention them using @name or @email. If they have the notification setting enabled they will receive a notification for the comment. 

For more information please view the Sharing and Collaboration Tools article from the Help Center and post any additional questions below.