A disk is a named entry in config/filesystems.php: a driver plus its settings. Code asks for one by name, Storage::disk('public'), and calls the same put, get, exists, url and delete whatever sits behind it, so moving covers to object storage is a configuration change.
The skeleton defines local, public and s3; FILESYSTEM_DISK picks the default. s3, scoped, ftp and sftp each need a Flysystem package, and a failed write returns false silently unless the disk sets 'throw' => true or 'report' => true.