How do I examine and reduce what "About This Mac" categorizes as "System" disk use? In "About This Mac" my Storage shows a System disk use of 419 GB. Since my storage is a bit tight and I'm gearing up to update to Catalina (from 10.14.6), I'd like to clear as much disk space as I can, and this huge chunk attributed to "System" seems a good place to start. 

What is in System? How do I examine the details of what's there? How can I delete (safely) some of the things there consuming disk space?



FWIW, I have a lot of local Dropbox files, use Google Drive, have a disk image on my drive, and have a bunch of Homebrew installed tools (in case any of these could wind up categorized as "System").
You should get a copy of DaiseyDisk or GrandPerspective. These apps will produce a mapping of your hard drive so you can check what is taking up so much space.