Querybased templates – A bit dense, can someone help me there?

Hi,

I have different templates as per category. I also want to display single posts using that same template. Apparently I need to use query-based templates for the latter. The code I found here: https://codex.www.ads-software.com/Theme_Development#Query-based_Templates

But from this point on I seem to be dense. Where exactly do I use this code?

And – I use Laila 2.0 (https://orangescale.com) – actually the main thing I want to do is change the header image according to category/posts in set category. Anyone know of an easier way for this than creating category templates for each category?