Where does apple keeps a copy of files when I turned off iCloud drive?

Check your home folder for a folder called iCloud Drive (Archive).

Apple support document says the following:

If you turn off iCloud Drive or sign out of iCloud, you have the option to keep a local copy of your files that are in iCloud Drive. Whether you decide to keep a local copy or not, a new Desktop and Documents folder is created in your home folder. If you choose to keep a local copy, your files in iCloud Drive are copied to a folder called iCloud Drive (Archive) in your home folder. Then you have the option to move any files that were in your iCloud Desktop and Documents, back to your new local Desktop and Documents.

Open Finder: Menu Bar → Go → Home

Should be a folder in there called "iCloud Drive (Archive)".