A block template is an HTML file with block comments

The file contains block markup, which is HTML with comments that carry the block name and its attributes, and nothing else.

<!-- wp:template-part {"slug":"header","tagName":"header"} /-->

<!-- wp:group {"tagName":"main","layout":{"type":"constrained"}} -->
<main class="wp-block-group">
  <!-- wp:post-title {"level":1} /-->
  <!-- wp:post-content /-->
</main>
<!-- /wp:group -->

<!-- wp:template-part {"slug":"footer","tagName":"footer"} /-->

A self-closing block comment is a dynamic block with no saved markup, which is why post-title and post-content have no HTML around them — they render at request time. The file is editable in the site editor, and the moment somebody does that the database copy takes precedence over it permanently, which is the next note.