Are you looking for a way to include a template file in the other files? While there’s probably a plugin for this, we have created a quick code snippet that you can use to include from theme dir in WordPress.
All you have to do is add this code to your theme’s single.php file or where you want to include a theme file or in a site-specific plugin:
<?php get_template_part( 'nav' ); // Navigation bar (nav.php) ?>
Make sure to change ‘nav’ with your filename and you don’t need to add .php with it.
Note: If this is your first time adding code snippets in WordPress, then please refer to our guide on how to properly add code snippets in WordPress, so you don’t accidentally break your site.