Look into the theme template hierarchy. There is no “about.php” file and you should not link to any of the theme (PHP) files directly. Instead, you would create a theme file such as “page-about.php” and create its content in wp-admin as a Page with the slug “about,” and WordPress will process that Editor content within the PHP file to create the final visible page. If you have pretty permalinks enabled you will want to link to “/about/” instead of the theme file, but because you have named the PHP file “page-about,” WP will apply it to a Page with the slug “about.”
