how to create a page that shows all of multiple category posts on a single page

I was seeing nothing in between the header&footer because I did not actually display the posts in my custom query, which can be done using template tags like the_title() and the_content(). More details here:

But then I found out that I could simply use the cat parameter in the URL to view posts in certain categories, e.g.,5,9 to view posts in the categories 1, 5 and 9.

So it was then easy to just stick the query URL in a menu item’s custom link, and I no longer needed to use the custom template.