MJ/templates directory OR to the child theme root?
All custom stuff should be child theme. It’s the purpose to avoid touching core files.
We have a MJ and a MJ-child folder on our ftp. (the child one has no folders other than framework)
That’s one of the main purposes of the Boiler plugin, so we don’t overlook important files when trying to create a child theme. You’ll need to figure which files are missing and/or needed if you’re going from scratch.
You say it is called ajax-modal-login.php
Take a look in the theme directory: /MJ9.