How to list posts from a sub-category that is listed in a parent category (more details below)?

I have an unusual situation I put myself in.

I’m making a custom theme and I want to display a list of subcategory posts with its own URL like this:

https://domain.com/category/sub-category

So far, https://domain.com/category creates and lists sub-categories as expected. When the sub-categories https://domain.com/category/sub-category is accessed, I get a 404 error.

Here’s my code for the listing of the subcategories:

    $cat_ID = get_category_by_slug( 'category' )->cat_ID;

    $categories = get_categories( [
        'parent' => $cat_ID
    ]);

    echo '<ul>';
    foreach ($categories as $item) :
        printf( '<li><a href="/categories/%s">%s</a> (%d Articles)</li>', $item->slug, $item->name, $item->count);
    endforeach;
    echo '</ul>';

Here’s the code for my subcategory listing its posts:

global $wp;

$slug = $wp->query_vars[ 'cat' ];

$cat_name = get_category_by_slug( $slug )->name;

if ( $cat_name != null) :

    $posts = get_posts( [
        'posts_per_page'    => -1,
        'category_name'     => $slug,
        'order'             => 'ASC',
        'orderby'           => 'title'
    ] );

    echo '<ul>';

    foreach ( $posts as $post ) :

        printf( '<li><p><a href="%s/%s">%s</a><br>%s</p></li>', $slug, $post->post_name, $post->post_title, $post->post_excerpt );

    endforeach;

    echo '</ul>';

The only way for the subcategory’s post is to properly display is to use the URL: https://domain.com/sub-category which is something I do not want.

So, how do I list the posts of the subcategories of the categories using the URL: https://domain.com/category/sub-category?