Apple took system protection to the next level in macOS 10.15 Catalina by splitting your normal boot volume into two pieces. It appears like a single volume on the Desktop, but it’s really two: one is labeled with the volume’s name, while the other has “- Data” appended to it.
The new main volume is read only and contains only operating system files. This doubles down on the previously added System Integrity Protection (SIP)feature, which prevented key files from being modified as a way of reducing the potential for malware exploitation. This new approach in Catalina is a more severe version of this.
The Data volume contains everything else, and is tagged in Disk Utility with a Home icon over its disk icon in the descriptive view. By using new APFS features in Catalina, Apple can create a “volume group” that links the system and Data volumes together as a single item in function, and use “firmlinks” that allow cross-volume symbolic links without duplicating files, further tying the two logically distinct items together. Bombich Software, makers of Carbon Copy Cloner, has a more detailed explanation on their blog. (And note that if you mount a Catalina drive in Mojave, both volumes mount correctly, but appear separately and unlinked.)
During the upgrade from a previous macOS to Catalina, Apple moves any files that are located in a path that can no longer be written by the user to the Home directory’s Shared folder in “Relocated Items.”
Read more at MacWorld.com