When you upgrade to macOS 10.15 Catalina, your boot volume will
effectively be split into two. Assuming it’s the standard internal
storage, your existing boot volume will be renamed to Macintosh
HD — Data, and a new read-only system volume created and given
the name Macintosh HD. However, when your Mac starts up in
Catalina, you won’t see the Data volume, as it’s hidden inside the
System volume, in what Apple refers to as a Volume Group.
Although new to macOS, this scheme is already in use in iOS, and
specifies the read-only system volume as having the role
APFS_VOL_ROLE_SYSTEM, and the writeable user volume has the role
APFS_VOL_ROLE_DATA. In that, the volume with the System role is
normally mounted at the root /, and that containing both user and
mutable system data is then mounted in /System/Volumes and
accessed from there using several firmlinks.
Nice explanation of a complex change in 10.15 Catalina.