Create Block Theme does not create file function.php, and that is one of the reasons why I use Child theme. This file should be added manually afterward.
Overall, FSE Child theme needs theme.json, even so with that file, I occasionally had a problem.
Plugin Create Block Theme creates a theme.json, and that is why you did not have a problem with Template parts.
I will link Support Threads from Neve FSE. This theme also has the same problem.
https://www.ads-software.com/support/topic/child-theme-problem-33/
“for PHP editing, it’s better to use a plugin.” well it’s not, but that’s not the topic of this discussion.
Through the Child theme, both PHP and CSS can be used directly from the file editor or with VS Code or similar software.
