When I go to Apply a site template --> From Microsoft section, I can see there are many site templates. From these many site templates, I can select any site template like "Brand Central" and apply to my site.
What is want here is, I have to create my custom site templates based on all existing site templates. Disable all the existing templates in section From Microsoft and add my custom site templates to From your organization section.
My problem is, I am not able to extract existing site script/site template and create my own.