Custom Page Templates are a specific type of template file that can be applied to a specific page or post. Our WordPress themes have more custom page templates. You can apply this custom page template to a page or post you want. On the editing screen of that page/post, find ‘Page Attributes‘ section, and you will find the “Template” drop down menu. Clicking on it will allow you to select a page template from available template list.
Select the page template you want from the list and update your page/post.
Read more about custom page template here: https://developer.wordpress.org/themes/template-files-section/page-template-files/