Which url points to which file

WordPress has a template hierarchy, he search down the files in your theme until he finds a suitable template for the content hes outputing following the hierarchy.

Here it’s a example from de Codex:

If your blog is at http://example.com/blog/ and a visitor clicks on a link to a category page such as http://example.com/blog/category/your-cat/, WordPress looks for a template file in the current theme’s directory that matches the category’s ID to generate the correct page. More specifically, WordPress follows this procedure:

1. Looks for a template file in the current theme’s directory that
   matches the category’s slug. If the category slug is “unicorns,”
   then WordPress looks for a template file named
   category-unicorns.php.
2. If category-unicorns.php is missing and the category’s ID is 4,
   WordPress looks for a template file named category-4.php.
3. If category-4.php is missing, WordPress will look for a generic
   category template file, category.php.
4. If category.php does not exist, WordPress will look for a generic
   archive template, archive.php.
5. If archive.php is also missing, WordPress will fall back to the main
   theme template file, index.php.

Here’s a image that shows the complete hirarchy:

He will follow this behaviour unless you rewrite it with the Rewrite API.

In your case house-facilities.php is a Custom Template, and household-instruments is a page that uses this custom template, go to your Admin -> Pages -> Household Facilities and edit it, on the edit page you will see in the right corner a block called Page Attributes, inside that block has a select called Template and you will see that House Facilities Index is selected.

So what happens? As described in the link above, if you want to create a template for multiple pages to use, you can create a file and register it as an template. To register a file as a template simply put a comment in the top of the file with the line, Template Name: My Template, now in the edit page, you can select that file to output any page as you want.

When I’m developing a theme I like to use the Show Current Template plugin to see which files the page is loading.

Too learn about the template hierarchy see the Theme handbook.