A great many people believe their files are safe because they are in cloud storage. Sync and backup are different operations and the difference shows up at the worst moment.
The distinction
Sync makes the cloud copy match your local copy. Delete a file locally and it deletes from the cloud. Encrypt your files with ransomware and the encrypted versions sync up.
Backup keeps a copy that does not change when the original does, and keeps versions from earlier points in time.
Most consumer cloud services do offer limited version history, typically thirty days. That helps and it is not the same as a backup.
The rule worth knowing
Three copies of anything you care about. On two different types of storage. One of them somewhere else physically.
That covers the realistic failure modes: drive failure, theft, fire, ransomware, and your own mistake.
What that looks like in practice
The original on your computer. An external drive that you connect periodically and otherwise leave disconnected – which matters, because a permanently connected drive gets encrypted along with everything else.
And an offsite copy, which can be cloud backup software rather than sync, or a second drive kept elsewhere.
The part people skip
Testing. A backup you have never restored from is a hypothesis. Restore a few files occasionally and confirm they open.
What to back up
Documents, photos, and anything you made. Not the operating system or applications, which can be reinstalled. That reduces the volume enormously and makes the whole thing manageable.