Hello AlanB2,
Unfortunately, there's no functionality to selectively restore Collections. Because Collections are only an association of Cases, Collections don't come over when a Case is restored.
One way this could be accomplished is to perform an Appliance (Node) Restore from the one instance into the other. This restore will bring over all Cases, Collections and Legal Holds as existing in the previous instance. In this way, you can restore Collections and Cases together while maintaining their association. Of course, this would require that the new instance to be able to access the Collection and Collection Set destination location. You'll want to delete the Case and Collection from the previous instance so that there aren't two instances accessing the same data. You'll also want to update the license on the new instance unless you've chosen to not restore the configuration and license as prompted during the Appliance (Node) Restore.
From there, if any other Cases, Collections or Legal Holds come over that you don't want, you can delete at will. I'd strongly encourage you to do this sooner rather than later to prevent two instances accessing the same data.
This is a very high-level suggestion. This method should only be employed if you know what you're doing with both an Appliance (Node) Backup and Restore.