For users on the Basic and Team plan, creating a thread of comments to enhance collaboration is possible. Sharing the document via the editable link can then allow free users to comment as well.
Highlight the text you'd like to comment on, and click the comment icon.
Write your comment and click the green "Send" button.
Click the highlighted text to open a comment thread. Click the three-dot icon to delete or edit your comment, or resolve the thread.
View all comments in a document
Click the 3-dot icon and then "View all comments". In the panel to the right of it, comments (if any) in the document will appear.