When working with WordPress themes, there are several standard and default template files that need to be included.
According to the WordPress.org developer documentation: “the only files needed for a WordPress theme to work out of the box are an index.php file to display your list of posts and a style.css file to style the content… Once you get into more advanced development territory and your themes grow in size and complexity.”
Required files for a WordPress theme to work:
The following is a list of what custom theme developers consider the standard/default files for developing WordPress themes from scratch:
“When you are building your theme, you will use template files to affect the layout and design of different parts of your website. For example, you would use a header template or template part to create a header” explains the WordPress team.
“The most critical template file is the index, which is the catch-all template if a more-specific template can not be found in the template hierarchy. Although a theme only needs a index template, typically themes include numerous templates to display different content types and contexts.”
Going much further into the specific requirements for a custom theme’s files really depends too greatly on the specifics of your organization’s website you are developing, and also on whether you will be utilizing a classic theme or a block theme development model.