Database dumps
A backup of the App creates a dump of the application database only.
It does not include Jira issue data (Jira backup is a separate process that creates a Jira backup file).
Jira backup vs App restore
Jira and BigPicrture store their data separately.
Creating a dump of the App data IS NOT a Jira backup.
BigPicture App Configuration > Advanced > Database dumps allows you to create (and download) a backup of the state of the BigPicture app.
How does a "restore" work?
An App restore overwrites current app data and replaces it with the state saved in the dump. There is no possibility of merging two different states of the app:
you can't combine two dumps
you can't amend an existing state