Here we won’t dive into the technical stuff. Suffice to say that classic themes and block-based themes are structurally different.

The image below shows the content of the Twenty Twenty-One Blocks folder:

You’d notice the experimental-theme.json file and the block-templates and block-template-parts folders.

The main difference between traditional WordPress themes and block-based themes is that block-based themes include templates that are entirely composed of blocks.

For example, open block-template-parts/header.html in your code editor. You should see the following code:

As you can see, the classic header.php template file has been replaced by an .html file containing several blocks.