When you design a template, you can pull data from different sources using dynamic tags.

When designing a post type template, we recommend picking a post to preview your template within the editor, ideally a post which contains all the possible data that you will utilize in your design.

To do that, you can go to Document settings (bottom left settings icon), then go to Voxel tab, and type the name of the post under Post to use in preview. Once you find and pick the post, press Apply changes. The editor will reload and use that post to preview your template.

In a similar fashion, when designing taxonomy term templates, you can select a specific term to use in the preview