Point Calibre to Folder

Hi all,
My computer died so I had to rebuild it. I have a back up of the folders/files but not an export. I thought there was a way to point Calibre to a folder and it would recognize it as a Calibre library of books. I use SyncThing to sync folders across computers. I've reinstalled Calibre and now I need to point Calibre to the folders where the books are but I can't remember how to do it. Everything I've tried wants to create a new empty library or import a library etc. I just want to point Calibre to the folder where the files are if possible. I have two libraries so the structure is
C:\Books\Mine
C:\Books\Hers
I thought I could point Calibre to the Books folder and it would see the libraries.
Any help would be appreciated
Thanks!