Local folders
A folder on this machine is a root like any bucket: pick it in Settings → Local folder with Add folder, or the + in the sidebar header, and it appears in the tree, on any disk or mounted volume. From the command line, oe folder add ~/data does the same.
Local roots are what make the copy and move story complete — a prefix dragged from S3 onto a local root streams through your machine and lands as files, and the other direction works the same way. A large upload goes one part at a time, and one cut off halfway continues from the parts already sent when you copy the same file again.
A page in your browser
On a page served by another machine, Add folder opens the browser's own folder picker, and the folder is kept by that page. In instant mode, at objectexplorer.com/app, there is no limit on these folders.
Streaming folders
Google Drive, OneDrive, Dropbox, Box and iCloud Drive can each keep a folder on your disk. Such a folder can be added like any other, and its files open in place. A file the sync app keeps online only is downloaded by that app when it is opened.
To reach those services without their sync app, see Google Drive, Dropbox, OneDrive and Box.
Running on a folder
npx @knockdata/objectexplorer ~/data opens on that folder and adds it as a root, so the next start finds it already there.
Next: Google Drive.