A local copy is always made. You can add, at the same time, a WebDAV space (Nextcloud, ownCloud, kDrive, Infomaniak), S3-compatible storage (AWS, Scaleway, OVH, Backblaze B2, Wasabi, MinIO), an FTP or FTPS server, and an SFTP server if the PHP ssh2 extension is present. The "Test connection" button writes then deletes a test file, and shows you the remote server's real error if it fails.