Where is the post loop?

Hi theme team,

thanks for the easy and convenient theme.
On our webpage we are using the “Simple Lightbox” plugin to show larger pictures when it is clicked on.
In our blog pages we automatically add the thumbnail picture under the headline. Currently it is not possible to click on this picture and therefore also not possible to see the picture in a larger scale.

I got in contact with the plugin developer to see if there is a possibility to get also this picture into the lightbox.
Here you can see the short discussion:
https://ww.wp.xz.cn/support/topic/lightbox-on-post-thumbnail/

There is also a detailed guide how to get this working: https://github.com/archetyped/simple-lightbox/wiki/Activation#display-post-thumbnail-in-lightbox

But unfortunately this is not just a function.php filter or similar but it needs ot be added to the “post loop”.
As discussed in the support forum this code snippet needs to be added to the desired template file in the theme (e.g. single.php, page.php, etc.).
But I cannot find the single.php or page.php in the themes folder. Can you tell me where I can find those files or where I would need to add the respective code snippet to get the lightbox working for the thumbnail pictures?
Most probably it would also be necessary to add this to a child theme to be compatible to some theme updates right? Can you advice a good guide how to set this up for Twenty-Tweny-Four?

Thanks in advance and kind regards

The page I need help with: [log in to see the link]