Commenting
If commenting is allowed, select text on the rendered doc to start a thread. Type the note, optionally your name, and submit. Reply in the thread. Resolve it when the point is done. Unresolve if you need it back.
If commenting is disabled, you can still read. You will not see a way to start threads. The rest of this page still works: live updates, theme, and the table of contents.
If commenting is private, you need to be the author or a teammate. Sign in, or open an invite the author sent you.
Anyone-commenting is only offered when the doc itself is anyone-visibility: anyone with the link can comment. That combination is the author's choice, not something you set in the browser.