According to the file structure of a classic theme, template files should be located in the root folder of your theme:
wp-content/themes/your-theme/[template].php
You can refer to the folder structure guidelines here:
Organizing Theme Files.
To override a template file, you need to create the same file in the child theme, in the same location as it is in the parent theme.
For more information on template hierarchy, refer to this guide:
Template Hierarchy.
Please note that only page templates can have custom templates, and they require the “Reading” settings to be set to a static page.