`.php` files are files that (that usually in general) get processed on the server. when they are processed, it is their output that gets sent back to the calling client (normally your browser). These can be either directly called, or included with other `.php` file calls.
`.html` files are files that (that usually in general) are just read and directly passed though as-is to the calling client. These are usually always directly just called.
WP themes are not something that directly get called by the visitor. WordPress is set up to (for most things) call the `index.php` in the root of the site, that starts all of the WordPress processing, that at some point, sees what theme you have set up, then includes the particular `.php` files from the theme directory to process.
Are you familiar with PHP?
`.php` files are files that (that usually in general) get processed on the server. when they are processed, it is their output that gets sent back to the calling client (normally your browser). These can be either directly called, or included with other `.php` file calls.
`.html` files are files that (that usually in general) are just read and directly passed though as-is to the calling client. These are usually always directly just called.
WP themes are not something that directly get called by the visitor. WordPress is set up to (for most things) call the `index.php` in the root of the site, that starts all of the WordPress processing, that at some point, sees what theme you have set up, then includes the particular `.php` files from the theme directory to process.