Laravel's notifications are sent to a notifiable. A notifiable provides configuration values that determine how notifications will be sent.
By default the package uses this notifiable class: \Spatie\Backup\Notifications\Notifiable. This class will read out the config file. All mail notifications will be sent to the mail address specified in the notifications.mail.to key of the config file.
If you use a channel that needs some get some extra information out of the notifiable you can easily extend the default notifiable.