How to get listed SharePoint templates under the organizational section

Hi @DevAsith ,
Custom templates (or third-party templates) made by your organization using site scripts will display in the site template gallery based on the type of site a user chose to start with - either a communication site or team site. Therefore, it is likely that all of the custom site templates will not appear in the site template gallery at once. Soon, users will have the ability to choose to view all custom site templates in the From your organization tab regardless of the type of site chosen.

Site templates provided by your organization will appear in the From your organization tab when selecting a template type. Customization instructions will vary depending on the site template design. You can create site templates to provide reusable lists, themes, layouts, pages, or custom actions so that your users can quickly build new SharePoint sites with the features they need.

To learn more about how to create custom site templates for your organization., you could refer to:
https://learn.microsoft.com/en-us/sharepoint/dev/declarative-customization/get-started-create-site-design

For more information, you could refer to this article:
https://support.microsoft.com/en-us/office/apply-and-customize-sharepoint-site-templates-39382463-0e45-4d1b-be27-0e96aeec8398#ID0EDBH=From_your_organization

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