I can mark a topic with a bookmark but I can't find where the bookmarks are kept. What am I missing?

I may have found it!
Click on your (logged in) user ID in the upper right between the magnifying glass & the hamburger, select User Control Panel from the drop-down list. That opens a new panel on the left where (under Overview) the third item is Manage Bookmarks. Your bookmarks then show and you can click on each to read them as well as check the boxes to manage them. I trust that this is what you were asking.

Well done Mr. Points! Thanks very much.