The is_page_template() function is for checking if the current page is using a specific custom page template. That being a template selected from the Template dropdown in the Page Attributes box when editing the page.
The WordPress conditional tags are not based around which template files are used. They are used to tell you what content is being viewed, regardless of which template files are present in a theme. For example, is_category() will return true when viewing a category archive, even if your theme does not have a category.php file.
The list of available conditional tags is documented here.
If you want to use Query Monitor to figure out what conditionals to use for what pages, use the Conditionals tab:
