I'm wondering if there's any type of backup service for Quickbooks online, whether it be built in or through a third party service.

QBO backs up daily or more often but is not user accessible. It only is restored if someone at the Amazon server pushes a button because someone compromised everyone's data.  There are third party apps that say they work to backup and restore which is an interesting concept since QB claims you can only import data complete as an overwrite (restore) within 60-90 days of subscription initialized. To get around that most be pure magic

there are two I know in the app store, check there

QBO does not have its own back up

https://apps.intuit.com/app/apps/home

the apps back up the file, but it can only be used to restore the file it backed up, it is not a stand alone viewer for a back  up