What function should I use to add a category as a menu to my WordPress theme?

Question

PHP

There are 5 pages in wordpress that use a function to
add_theme_support('menus');automatically create menus from page titles. And what do you need to write so that you can add a rubric to this menu?
Going into the menu settings, creating the "Main Menu" so that I don't add it there, all the same, only the pages that I create by default are displayed. And I want to add to this menu, for example, a category ..
from existing pages in my theme there is:
category.php
style.css
home.php
single.php
header.php
functions.php
index.php
footer.php