CobaltHaze I am not a software person, but I've been using the app for three years—this is my experience.
Sync and backups are two different things. Syncs can be set up automatically to update your content across multiple devices (or you can change it to manual if you don't like how it will randomly sync when you're in the middle of writing an entry, like me). However, if you need to move the entire database to another device, you can create a manual backup in Settings (per the FAQ).
Yes, in OneDrive, the sync folder is in Apps > Diarium. I generally don't touch that folder as I let the Diarium magic do its thing. I would recommend the same, as Timo did.
As sidenote for syncing, if you are using Diarium on multiple devices, I'd recommend making sure both devices are synced with the most recent data before making further updates. If you make a new entry/edit on one device and then make an entry/edit on another device, and neither device has been synced, the sync may combine the entries from both devices, but sometimes one may be overwritten. And that has made me sad. So, I always make sure I sync entries on one device before moving to another device (e.g. my phone to my computer).
When you create an actual backup from Settings > backup, the backup file includes the .diary extension (in contrast to the actual sync database/index files that start with V4), and any attachments added will be stored in a folder also starting with V4 and files inside have no visible extension. So that info may help you know what file types you accidentally deleted/restored.
The sync is one ongoing file; backups can create a "version" with the most recent data at the time of backup. If you import the backup (which you don't need to do unless you are importing it to another device), the import will be older than the latest synced file. I like to create a backup once a week just as a precaution in case something happens to my synced file. (I only keep the latest backup to save on space.) The OneDrive Diarium folder does contain a Backup folder where you can put them, but I add my backups to iCloud so I never need to touch my OneDrive Diarium folder at all.
