I want to restore an old backup onto my iPhone. Will that delete the data currently on my iPhone, or does my data get added to what’s in the backup?
I would say, you can’t restore a device until you erase the iPhone so the restore doesn’t really erase anything - just that you have to pass through erase to get to restore in the first place.
So I realize my argument is close to "splitting hairs", but I'd say: Restoring a backup implies everything is erased from the phone - you can’t mix a current set of data with a restore
When you restore your iPhone, whether it's from backup or as a "new" iPhone, all data on the phone since the last backup that was created will be erased (gone).
If you want to restore your iPhone from backup, the best thing to do is take a backup first using either iCloud backup or through iTunes, then perform the restore process.
Once the phone goes through the setup screen, you can restore from backup or start fresh.
I just did a restore and the data saved in the file sharing section of the applications was not removed so the accepted answer is wrong. It might have deleted the data if the app didn't exist before but I didn't try. The software version was not changed. As far as I can tell it just reset a bunch of settings.