Mac hard drive clogging up despite catalogue being stored on external hard drive

Another thing to watch out for when importing is to make sure that the Make a Second Copy To option is either deselected, or if selected make sure it’s pointed to the folder you really want. If it’s always making second copies of all imports into a folder on your computer, that could be a reason you’re losing storage space.

You haven't mentioned what size your internal disk is and how much space it has remaining, which can be helpful to those of use trying to provide help. Additionally, when LrC imports raw photos it creates cache files for use in the Develop module. By default, these are stored on your primary disk and the default size for the cache is 5GB (see atached screenshot). The location and size can be changed by the user, but don't reduce below 5GB. In some cases, users may have increased the cache size to improve performance. If you've done so, then reduce to 20GB or less. 

The previews are saved in the same folder (well, actually in a subfolder of) the folder that contains your catalog file.

The photos are saved wherever you tell Lightroom Classic to save them when you import.

So, your task, confirm (rather than just say) that the catalog and previews are on an external drive, and not on the internal hard drive. Confirm (rather than just say) that your photos are saved on the external drive and not on the internal hard drive.