Wow, Catalina really mixes things up with regard to the file system layout. It also introduces something called firm links, which are not symlinks nor hard links, but something akin to a union mount of two folders. This is used, for example, on the /Applications folder.
This folder shows the contents of user installed apps, as well as SIP protected apps that actually live in /System/Applications.
On my system, the SIP protected apps are the standard Apple-installed apps:
App Store, Automator, Books, Calculator, Calendar, Chess, Contacts, Dictionary, FaceTime, FindMy, Font Book, Home, Image Capture, Launchpad, Mail, Maps, Messages, Mission Control, Music, News, Notes, Photo Booth, Photos, Podcasts, Preview, QuickTime Player, Reminders, Siri, Stickies, Stocks, System Preferences, TV, TextEdit, Time Machine & VoiceMemos
You can find more info here:
What used to be Macintosh HD is now mounted on /System/Volumes/Data and this is where all your user modifiable data lives. Just about everything else is spread across a number of individual read-only file systems, with the exception of /var/vm that is a read-write file system, but they are all different APFS partitions.
There is some additional, and very good, information here as well