How to move page template files like page-{slug}.php to a sub-directory?

I want to move page template files like page-{slug}.php to a sub-directory within my theme, in a way that WordPress will recognize them automatically. If the page templates of the said form doesn’t exist within the sub-directory, then WordPress should fall back to the default template loading rules. How can I achieve that?

Making images from single.php pointing to an attachment .php template

I am trying to make the inserted images from a post (single.php) to point to (single-attachement.php) an attachment page. So far I checked a post here on stackexchange and added this code on the top, before the header of single.php, just that it’s showing me a link to the last image on the page, instead of modifying the structure of the image link.

Creating Ajax backend for Specialised Page Template? Should admin-ajax.php be used?

We’re scoping an upcoming project that will require the development of a large php web application within an existing wordpress site – this is for our use only and we’ve no intention of trying to package it for other users; so interoperability isn’t really a concern as long as we can continue to apply WordPress updates as they’re released without breaking our application.