How do template differentiate “views”?

Hi,
I’m having a little problem about installing features that should show only on certain views:
Category pages /individual posts. I’ve been reading about the “loop” but even thought I think i’m going to learn the code by memory I cannot figure out how does wp know when I’m seen the home page, a category page or an individual page.
for example, I’m trying “next-previous” in category from Scripty Goddess (https://www.scriptygoddess.com/archives/2004/07/24/nextprevious-posts-in-same-category/)
I’ve uploaded the plugin and activated it.
but now, where on earth do I put the code so it would show only on individual posts?
Thanks ??