Echo get_the_category() outside of loop (global?)

I realized that the_title(); can parse in file1.php outside of the loop, so why can’t the category and ID? The following solved my issue.

// file1.php
<div>
    <h1><?php the_title(); ?></h2>
    <?php
        $category_array = wp_get_post_categories($post->ID);
        $category_list = array();
        foreach ( $category_array as $categories ) {
            $category_list[] = get_cat_name( $categories );
        }
        $lister = implode(' • ', $category_list);
        $list_categories = $lister;
        echo $lister ;
    ?>
</div>

// file2.php
<article>
    <?php
        if ( have_posts() ) : while ( have_posts() ) : the_post();
            the_content();
        endwhile; endif;
    ?>
</article>

Alternatively, to pass the category link, you could use the following (warning, changed variables below):

<ul>
    <?php
        // get post categories
        $cats = wp_get_post_categories($post->ID);
        foreach ( $cats as $category ) {
            echo "<li>";
            $current_cat = get_cat_name($category);
            $cat_link = get_category_link($category);
            echo "<a href="https://wordpress.stackexchange.com/questions/231734/$cat_link">";
            echo $current_cat;
            echo "</a>";
            echo "</li>";
        }
    ?>
</ul>

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