Built-in templates
Pages that the system generates dynamically — article detail, product detail, course sales pages — get their appearance from here.
Why templates exist
You're not going to lay out each article's detail page one by one — that's the template's job. Configure it once and every article uses it.
Each template has its own options (show tags or not, card spacing, what goes in the sidebar), and can also be rewritten wholesale.
Question: Does editing a template affect already-published content?
Yes, and immediately — every page using that template changes together. That's deliberate: the whole point of a template is that you only change it once.