There's nothing too esoteric about a calibre library. It ia a folder that has a metadata.db file in it, along with Author and Book Title subfolders, the latter containing a cover image, a restoreable copy of the book's metadata, and usually one or more book format files -- EPUB, PDF, CBZ etc -- but only one of each format.
The export/import feature was implemented to address this question ==>>
How do I move my calibre data from one computer to another?. And then there's this question ==>>
How do I backup calibre?
Basically you want to keep a copy of the library folder and everything it. The folder/file synch utilities maintain such copies incrementally. After the first copy, which would be of everything, they only copy changes (adds, changes and deletes) since the last copy.
Because the backup is an exact replica of a calibre library, it in itself is a calibre library. So if your library is destroyed, you can us normal OS tools to copy the backup to restore it, or even point calibre at the backup library and use it directly.
I use GoodSync to backup all my data, including calibre libraries and configuration data, to external USB 3.0 drives on a daily basis, and to a remote FTP server on a weekly basis.
BR