You have one current master catalog file with the file extension ".lrcat". All of the other files with that extension are older catalogs or backups of your master catalog file. If you have one and only one master catalog (and you should) then the other files should be either in a Backup folder for the time when your master catalog file gets corrupted or can safely be deleted. Along with that master catalog are two folders the use the name of the master catalog plus additional extensions to that name of "Previews". and "Smart Previews". These folders are also given an extension of ".lrdata" . I see at least one file named "Lightroom Catalog.lrcat". Associated with that master catalog file are two preview folders "Lightroom Catalog Previews.lrdata"
and "Lightroom Catalog Smart Previews.lrdata". It is important that these folders stay together with the master catalog, but as I will indicate later Lightroom will recreate Previews as needed and these particular folders are probably surplus.
The files with the extension of ".dmg" are disk image files (dmg) used to install a particular version of Lightroom. You can delete all of these except the latest one if you ever want to reinstall Lightroom 6. Since LR6
The Previews folders will recreate themselves. And over time, these have accumulated previews that you may never need again or can be recreated on the fly the next time you access that image in Lightroom. Unless you need to use Lightroom on a laptop separate from the master image files on a separate external disk, you do not need to a Smart Preview folder at all. You can instruct Lightroom to not create smart Previews in the import dialog. At this point, I would suggest that you delete both Previews folders and let LR6 rebuild the standard previews that you need as you open these files the next time in LR6.
As to the other files named ".lrcat", instead of deleting these, I would suggest that you move them to your Backups folder just in case one of them has something that you needed and should be retained. Also Lightroom defaults to creating a Backup folder to store your catalog backups. Initially this folder is stored as a subfolder of the Lightroom Folder because Adobe is not aware of any other external drives. You can move your bacup folder to and external drive to free up space on this drive. And when you exit Lightroom and the Backup dialog pops up, you can specify this backup folder on the external drive so as to not write backup in the drive containing Lightroom.
Additional space saving measures that you can take include moving ALL of your image files to an external disk volume and not take up ypur primary disk with images files. Lightroom is happy to access your imported image files if located on a different volume from the volume containing the catalog folder.
