A few more details that I've managed to suss out. The Data volume is mounted on /System/Volumes/Data. Currently, there are 21 firm links into it. They are enumerated in /usr/share/firmlinks (included below).

    /AppleInternal Device/AppleInternal
    /Applications Device/Applications
    /Library Device/Library
    /System/Library/Caches Device/System/Library/Caches
    /System/Library/Assets Device/System/Library/Assets
    /System/Library/PreinstalledAssets Device/System/Library/PreinstalledAssets
    /System/Library/AssetsV2 Device/System/Library/AssetsV2
    /System/Library/PreinstalledAssetsV2 Device/System/Library/PreinstalledAssetsV2
    /System/Library/CoreServices/CoreTypes.bundle/Contents/Library Device/System/Library/CoreServices/CoreTypes.bundle/Contents/Library
    /System/Library/Speech Device/System/Library/Speech
    /Users Users
    /Volumes Device/Volumes
    /cores Device/cores
    /home Device/home
    /mnt Device/mnt
    /opt Device/opt
    /private Device/private
    /sw Device/sw
    /usr/local Device/usr/local
    /usr/libexec/cups Device/usr/libexec/cups
    /usr/share/snmp Device/usr/share/snmp