When and Why is is_singular(‘my_cpt’) true while in_the_loop() is false?

I’m using example code for a templating system.

Page at this address: http://project.test/my_cpt/hello-post/.

Cannot understand why is_singular( 'my_cpt' ) is true while in_the_loop() is false.

Within the page template, it seems like The Loop “works”:

I guess my question is when are is_singular() && in_the_loop() both true?

And when I run if ( have_posts() ) { while ( have_posts()... is that in the loop or is it creating the loop?

Partly it has to do with which filter is being hooked in the function/method which performs the if && test.

The example above is being used with the template_include hook, which is getting run for pages, posts and maybe even links, menus, media, etc. So that would be why both tests are necessary.

The filter looks like:

add_filter( 'template_include', array( __CLASS__, 'my_template_include_method' ) );

or if it was not calling a class method would just be:

add_filter( 'template_include', 'my_template_include_function' );

The full method/function looks like:

The wpbp_get_template_part is from a plugin boilerplate I recently discovered called WordPress Plugin Boiler Plate.

Since I’m looking, at the moment, a singular post, I can hook into single_template like this:

add_filter( 'single_template', array( __CLASS__, 'my_single_include_function' ) );

It looks like this:

The templates themselves look like: templates/content-my_template and templates/single-my_template.