The idea
Block themes use block markup for templates and template parts. A template defines a page layout, while reusable parts such as a header can appear in many templates. The theme.json file supplies shared settings and styles. Changes saved in the Site Editor may be stored in the database and can override theme files, so distinguish file changes from editor changes during testing.
Worked example
A fictional theme uses templates/index.html for its fallback layout and parts/header.html for navigation. It sets typography and a small color palette in theme.json. Updating the header part changes the shared structure, while a single-page template can customize one page type.
Try it
Sketch a theme file tree with a fallback template, a page template, a header part and theme.json. Describe which file you would change to update shared typography and how you would check for a saved editor override.