You would use either the zotmoov or attanger plugin. After entering your "move to" folder in the settings, you right-click and select the move option. Normally that is a single folder (but subdirectories are also possible). Best to do that move in small chunks. Future attachments downloaded via the Zotero web connector will be saved automatically to that folder. https://github.com/wileyyugioh/zotmoov https://github.com/MuiseDestiny/zotero-attanger
https://github.com/wileyyugioh/zotmoov
https://github.com/MuiseDestiny/zotero-attanger
Be aware of the Zotero file renaming that will also occur.
https://www.zotero.org/support/file_renaming
Note the limitations of linked files.
https://www.zotero.org/support/attaching_files#linked_files