Themes that can be used on WordPress appear in the Appearance>Themes section on the WordPress CMS panel. 3 theme versions come with the installation of WordPress, these themes are regularly updated by WordPress.
Apart from these themes, you can upload new paid or free themes to the WordPress panel. Or you can code a theme yourself and use it in WordPress infrastructure.
Template files created by WordPress are used for WordPress theme creation. When the theme is created, the theme created in the Appearance>Themes area appears and the theme can be activated and broadcast.
The theme is created using the theme files of WordPress under the created file. In order for the created theme to appear in Appearance>Themes, the following 2 files must be indexed;
Without these files, the new theme option does not appear in the theme change area, it gives a warning in this area because there is a missing file.
WordPress infrastructure is a structure built with PHP, so PHP is used when creating theme files. HTML, CSS and Javascript are used in the design of the theme.
Libraries such as Bootstrap and Jquery are also widely used. Even in WordPress, the Jquery library comes by default. You can use the jquery library or you can choose to use Javascript from the theme for your needs.
The following template files were created by WordPress.
Source:
https://developer.wordpress.org/themes/basics/template-files/