Using Private template URL

Once you generate the custom tempalte using Azure DevOps Demo Generator - Template Builder, you can use the same template as a master copy to provision the sample projects.

Provisioning your project from your custom template using public URL

Using private template URL you can provisoin the project. You have multiple options here

Using the template URL from public GitHub repository, refer Provisioning the project from your custom template
Use the template URL in the home page of Azure DevOps Demo Generator with querystring parameter ?templateurl. This supports only the public URL

Ex: https://azuredevopsdemogenerator.azurewebsites.net/?templateurl=public_url

Once you format the URL, press enter to reload the page.
Click on Sign In button
Once you navigate to Create Project page, you should see the private tempalte selected by default
Select the Organization. Provide the Project Name. Choose Create Project to start provisioning a project

Note: Once the process completes, the template will be discarded by the system. If you want to reuse the tempalte, you can use the Private option in the Choose Template dialog box

You can refer the document Provisioning your project from your custom template