The catalog file has no thumbnails or accompanying photo's. There is a file ending with .Previews.lrdata that contains previews of the image files that have been imported into the Catalog. The Catalog File .lrcat (LRCAT) does not contain any image files it just records info about your image files and the work you have done with them in Lightroom. See the image capture. Have a look at the video tutorial on how the LR Catalog works at the link below.
The Lightroom Catalog | George Jardine on Lightroom | Adobe TV