Yes, the WP template hierarchy can be one of the difficult things to understand. I’ve tried to explain archive pages to clients and even to the people I work with/for. “I can see the page on the front end, why can’t I change the content?”
Anyway, if you are wanting to show the list of case studies on the front end than the template you mentioned, well I don’t know what file archive-custom.php is. If you’re custom post type is case-study then I would expect it to be archive-case-study.php. Is archive-custom.php a file that is supplied by the theme you’re using?
What do you want to do with the archive template file that it’s not doing.
