I noticed that when I changed the WP_Query to ask for a certain category, the page served was category.php. So I’m guessing there’s a block of code somewhere that decides which template to serve based on keys in the query. Where is it?
How is WP_Query parsed to determine which template to request?
The template loader determines which template to load for a given view, there’s a filter available to change this though,
template_includeas seen on the source page linked.Moved from comment to answer.