Catalina: Purpose of /System/Volumes/Data/Previous Content/?
I think it was originally used for Archive and Install macOS installs.
However, you haven't been able to select a Archive and Install for ages.
To me it looks like it is continuously updated in real-time which makes me think its actually linked to the original sources and the files you see are the actual files not copies. It also seems to be well hidden from the file system. Perhaps left there for comparability reasons.
Perhaps look at your drive using DaisyDisk to find where the space is getting used. macOS allows purge-able space take up to 80% of the drive, however, it should automatically reclaim when the storage is required.
https://daisydiskapp.com/manual/4/en/Topics/PurgeableSpace.html?source=PurgeableSpace&amp;appEdition=Standard&amp;appVersion=4.10&amp;osVersion=10.15.5