They aren’t in a public repo but they are stored on our public CDN.
A template map is hosted at https://forge-templates.us-west-2.prod.public.atl-paas.net/templates.json
Templates are hosted at <template_directory_name>.zip for example confluence-content-action-custom-ui is accessible at https://forge-templates.us-west-2.prod.public.atl-paas.net/confluence-content-action-custom-ui.zip