There are some apps that can do it.You can't. You don't have access to the backup like you do with an iTunes backup. Only thing you can do is restore the backup to your device, then take what you need off, or create an iTunes backup afterwards and extract it from there.
Interesting, this must be relatively new, because it sure didn't exist this time last year. Needed it as well.There are some apps that can do it.
https://toolbox.iskysoft.com/iphone-recovery/extract-app-data-from-iphone.html for example.