macOS Catalina mounts the read-only system volume at / and the read-write data volume at /System/Volumes/Data

mount.txt

% mount
/dev/disk1s6 on / (apfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
/dev/disk1s5 on /System/Volumes/Data (apfs, local, journaled, nobrowse)
/dev/disk1s4 on /private/var/vm (apfs, local, journaled, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)

% ls /System/Volumes/Data
Device	Users

% ls /System/Volumes/Data/Device
Applications	System		Volumes		home		opt		sw
Library		Users		cores		mnt		private		usr

# These are mostly the apps I have installed on this OS.
# I guess the presence of Safari means that it can be updated separately from the OS?
# Utilities is empty.
% ls /System/Volumes/Data/Device/Applications 
1Password 7.app		SF Symbols.app		Utilities
Pastebot.app		Safari.app		Xcode-11.0-beta.app

% ls /System/Volumes/Data/Device/Library       
Apple			Extensions		MessageTracer		Scripts
Application Support	Filesystems		Modem Scripts		Security
Audio			Fonts			OSAnalytics		Speech
Caches			Frameworks		OpenDirectory		Spotlight
ColorPickers		GPUBundles		Perl			StagedDriverExtensions
ColorSync		Graphics		PreferencePanes		StagedExtensions
Components		Image Capture		Preferences		StartupItems
Compositions		Input Methods		Printers		SystemExtensions
Contextual Menu Items	InstallerSandboxes	Python			SystemProfiler
CoreAnalytics		Internet Plug-Ins	QuickLook		Updates
CoreMediaIO		Java			QuickTime		User Pictures
Desktop Pictures	Keyboard Layouts	Receipts		User Template
Developer		Keychains		Ruby			Video
DirectoryServices	LaunchAgents		Sandbox			WebServer
Documentation		LaunchDaemons		Screen Savers
DriverExtensions	Logs			ScriptingAdditions

% ls /System/Volumes/Data/Device/System                
Library

% ls /System/Volumes/Data/Device/System/Library 
Assets			Caches			PreinstalledAssets	Speech
AssetsV2		CoreServices		PreinstalledAssetsV2