Where does Google Drive File Stream store offline files and folders on a Mac?
By default, Google Drive for desktop (formerly known as Google Drive File Stream) stores cached files in ~/Library/Application Support/Google/DriveFS/$account_id/content_cache, where $account_id is the numeric ID of your google account. The files aren't encrypted or anything, but they don't have any metadata (or even their original filenames) so I don't think you'll find them terribly useful. The metadata seems to be stored in various SQLite databases in in the $account_id folder. In an emergency an expert might be able to reconstruct the original file names and folder structure.
If you want a full backup of your Google Drive, you can try backing up the Google Drive volume itself (/Volumes/GoogleDrive), while Google Drive for desktop is running.