Is cloud file sync, such as OneDrive or Dropbox, a backup?
Not on its own. Sync tools are designed to keep every copy identical, so when files are deleted, overwritten or encrypted by ransomware on one device, the change is copied to the cloud and to every other device within a few minutes. Most services keep a version history and a recycle bin, and some offer to restore a whole library to an earlier point in time, which can save you after an incident. But that history has limited retention, it is controlled by the same account the attacker may have taken over, and nobody tests it like a backup. Treat sync as convenience and collaboration. For data that matters, add a real backup: a separate copy, under separate credentials, with retention you choose and restores you have tested. That applies equally to file servers that sync to the cloud.