How to create category page

You’ll want to stick something like this in your theme’s category.php file:

<?php
// fetch the categories with the parent being the current cat
$sub_cats = get_terms('category', array(
    'parent' => get_queried_object_id()
));
if(!is_wp_error($sub_cats) && $sub_cats)
{
    foreach($sub_cats as $cat)
    {
        // Create your grid here, this is just an example
        printf(
            '<a href="https://wordpress.stackexchange.com/questions/57529/%s">%s</a>',
            get_term_link($cat, $cat->taxonomy),
            $cat->name
        );
    }
}