How Clean Re-installs Change in Catalina
Because you want to re-install macOS, the logical thing to do would be to wipe the System volume, which suggests that you could get away with retaining your own files on the Data volume through a clean re-install. Sadly, that’s wrong.
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To perform a clean re-install in Catalina, once in Recovery Mode, you need to wipe your Data volume, that’s the one named
Macintosh HD - Data, or something similar if you are using a custom name. There are two ways to do this in Recovery Mode: you can select the volume at the left of Disk Utility’s window then click on Erase, or you can select the volume and use the Delete APFS Volume command from the Edit menu (a shortcut to this is to click the – tool).[…]
At one time, the Recovery volume contained sufficient to restore the current version of macOS if you have entered ‘local’ Recovery Mode using Command-R, but this doesn’t seem to work any more. Whichever type of re-installation you have set now requires that version of macOS to be downloaded afresh.
