Great question! Arc doesn't have Bookmarks – we used Pinned Tabs instead.
Pinned Tabs work similarly to Bookmarks. They're easily accessible at the top of your sidebar and can be returned back to their originally pinned URL by clicking their icon!
Just like Bookmarks, Pinned Tabs can be stored in Folders. Just hit Command-T ... and type "New Folder" to create a Folder for work websites or articles you want to read.
You can read more about Pinned tabs here.