In addition to building a Cloud Assembly template from a blank canvas, you can take advantage of existing code.
Cloud template cloning
To clone a template, go to Design, select a source, and click Clone. You clone a cloud template to create a copy based on the source, then assign the clone to a new project or use it as starter code for a new application.
Uploading and downloading
You can upload, download, and share cloud template YAML code in any way that makes sense for your site. You can even modify template code using external editors and development environments.
Integrating Cloud Assembly with a repository
An integrated git source control repository can make cloud templates available to qualified users as the basis for a new deployment. See How do I use Git integration in Cloud Assembly.