How does wordpress know if a page is an archive?

Hi all,

I’ve recently started working at a company whose website was build using wordpress on a completely custom theme. My archive pages don’t work and only pull up an exact replica of the blog. There aren’t very many files in the theme folder (index, functions, comments) so there’s no header/archive/single/page-templates, etc. and everything is compiled in index.php.

There is a section in index that says if (is_archive()) {…} but none of the code inside is being executed, which leads me to believe that wordpress doesn’t know that these pages are archives and thus are treating them like the main blog page.

I feel like this may have something to do with WP_query but unfortunately after reading through the codex I still don’t really understand how it works. Does anyone have any insight?