WordPress doesn’t work that way. In WordPress, none of the posts or pages that you visit actually exist anywhere on the disk. The content is grabbed from database and then the template files are filled with them and sent to browser.
In your case, you should create a file name about-us.php instead of about-us.html and then include it in your template by using this:
get_template_part('path/to/this/file/about-us.php');
Now you will be able to create a page from admin panel and use this file as its template.
As for your questions about link… Again you shouldn’t directly link to a PHP file, because it simply won’t work. If you access a PHP file directly, it doesn’t load WordPress’s engine when it is loading.
To do so, you should use the slug. Create a page, write down its slug, and then create a dynamic link in your PHP template like this:
<?php echo site_url('/some-path/slug-goes-here'); ?>
The above function with append /some-path/slug-goes-here to your site’s URL and output it, which would be your page’s link.