During the core editor and customize meeting on May 28th, it was clarified that templates and template parts will likely be PHP files instead of HTML files.
The background to this is the need for dynamic urls and the ability to translate text strings.
PHP files will be tested and evaluated before a final decision is made.
This is also one of the reasons to why I postponed releasing the course; I do not want to spread misinformation.
These lessons will be updated once we have a decision, but this is the format that works right now, with Gutenberg version 8.2 – 9.4+, and it is the format that you can test right now.
Discussion: What file-type should FSE themes use?https://github.com/WordPress/gutenberg/issues/27144
//Carolina, May 30 2020. Updated November 25th 2020.