I have not used the Activity Timeline in the past and would like to start using it to review things I may have missed following up on. I can't seem to even find it. Where do I see it? How can I add it to Favorites once I do?

No, the activity timeline is for record pages.

You could go to the Task tab (search for it in the app picker or you may have a Task componennt usually on your home page where you can click "View All") though to see all the Tasks. There are separate list views (ie Overdue, today's tasks, etc) where you could see them all