Updating library folders: showing a new subfolder

There is an option "Add Folder" which makes a given folder visible that would not otherwise show, as a result of the locations of the imported images. That might be useful if you then plan to virtually drag-drop images' locations within Lr Classic Folders panel, whereupon LrC instructs the operating system to implement those moves for real.

The catalog holds "as recorded" file path information for all the imported images, and that is what the folders panel is referring to - you are effectively seeing the output from a database query. The actual file storage is NOT the source for what this panel shows; that storage does not even need to be currently accessible.

At root the way to alter what the folders panel shows, is to update what file path information the Catalog has got recorded against the respective images. Importing images causes new images plus their containing folders to show, but folders will not otherwise show merely because they exist on disk (unless with a manual "Add Folder", or "Show Parent Folder"). Images will never show, if they are not imported. When the arrangements of the storage are externally changed behind LrC's back, then this introduces a mismatch. That mismatch can be cleared up by informing the LrC Catalog of that external change.

Analogy: say you move house but don't tell anyone, people and organisations will continue to (try to) contact you at your old address, and in good faith will continue to list you as still living there, knowing no different... until you inform them of the change. That is in the nature of records.

To the extent we can understand and treat the Catalog in these terms - as paperwork ABOUT all these images and folders, rather than a view of those directly - so much the better, and the more intuitive.