There are several kinds of comments you can leave students in Classroom. Knowing how each one works can make you more efficient and effective.
Adding class comments: Do this by adding a comment in your class stream on the “outside” of an assignment or announcement. This will make the comment visible to the entire class (important if it’s an answer to a question anyone might have).
Adding private comments: Do this by viewing student results and clicking on an individual student. On the right, where you can see student submissions, the comment bar at the bottom adds a comment that only the student can see (important if it has sensitive grade or feedback information).
Adding comments in a doc/slide/sheet/drawing: Do this by clicking on the student’s file that he/she submitted to you. Click the black speech bubble icon after highlighting something you’d like to comment on. This adds a a very pointed comment on specific items in student work (important to be very exact in feedback).
https://support.google.com/edu/classroom/answer/9093530?hl=en