Suppose I want to have a note which points to a notebook. (Say a note which I update with notebooks that are active at present.)
I can go to the notebook, create an external link, store that in a directory in the file system, and have my note point to THAT - but I cannot see how to have a note use the "external link" of a notebook to switch to that notebook.
[It DOES work to make a directory in the file system which has a link that is the external link for the notebook, and then put file:///c:\links\somelink.url as url into the note. Which switches to the notebook of interest, but depends on bouncing to the pc filesystem to do it. Kind of weird...]
Here I've got "Copy Notebook ID" as well as "Copy external link" on the Notebook context menu (can't remember if that's built-in, or provided by a plugin?), and this format works:
[Shortcut description](:/@xxxxxxxxx)
where xxxxxxxxx is the copied Notebook ID.
Comparing that to a link to a note:
[Note Title](:/xxxxxxxxx)
you can see it is the at sign, @, that specifies it is a notebook, and just like using @ in Go to Anything to find a notebook.
(Checked, and found the additional context menu item "Copy Notebook ID" is provided by the Templates Plugin)
OK, in 2.12.17 the template plugin causes "copy notebook ID" option to appear, but a link structured as johano writes does not work (opens file explorer instead....)