MacOS now splits the operating system and data files into two volumes, but they should never appear on the desktop.

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.
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https://www.macworld.com/article/3451363/why-you-might-see-a-new-data-disk-in-catalina.html