I would like to be able select an item in Devonthink and share its URL directly with other applications (e.g. Todoist) in a way that allows me to click the link and arrive at the correct item in Devonthink.
Data > Copy Item Link puts the link on the clipboard.
You might assign a memorable keyboard shortcut to that action.
The Share sheet is also available from this arrow icon on a document’s toolbar.
You would want to speak to ToDoIst about their share behavior for accepting inbound messages.