That "config" as you are calling it is stored on flash as a template. The template is used to fill in the form on the Add Container page. Apps on the Community Apps page know about templates that have already been created by the docker authors, and that is how it is able to help you install a new docker.
But anytime you use the Add Container page, whether for one of the Unraid supported dockers from Community Apps, or for something on dockerhub, the settings you make on the Add Container page is stored as a template on flash and it can be reused to get those same settings for the Add Container page.
So even those docker hub containers can be setup with the same settings as before.