Object storage has no real directories and no way to change single bytes; each object is written and read as a whole via its key. In return it scales almost without limit and is cheap per gigabyte. S3-compatible options include MinIO, Ceph RGW, Hetzner, IONOS, Wasabi and Backblaze B2.
Nextcloud can use S3 as primary storage. Files then sit in the bucket as numbered objects without readable names; the mapping exists only in the database. Without a database backup the objects are practically useless.
Typical mistakes: wrong endpoint or region, missing use_path_style on self-hosted systems, and bucket policies that accidentally allow public read access. Versioning and Object Lock protect backups against deletion and ransomware.
Example
aws s3 ls s3://backup-bucket --endpoint-url https://s3.example.com