WordPress: Related posts with WP_Query. https://since1979.dev/wordpress-related-posts-with-wp_query/

related-posts-complete.php

<?php
$related_query = new WP_Query(array(
    'post_type' => 'post',
    'category__in' => wp_get_post_categories(get_the_ID()),
    'post__not_in' => array(get_the_ID()),
    'posts_per_page' => 3,
    'orderby' => 'date',
));
?>

<?php if ($related_query->have_posts()) { ?>

    <div class="related-posts-grid">

        <?php while ($related_query->have_posts()) { ?>

            <?php $related_query->the_post(); ?>

            <div class="grid-item">

                <a href="<?php the_permalink(); ?>">
                    <?php the_post_thumbnail('post-thumb-small'); ?>
                </a>

                <h5>
                    <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
                </h5>

            </div>

        <?php } ?>

    </div>

    <?php wp_reset_postdata(); ?>

<?php } ?>


<style>
.related-posts-grid {
    display: grid;
    grid-template-columns: 1fr 1fr 1fr;
    grid-gap: 40px;
    max-width: 1000px;
    margin: 60px auto;
}
.related-posts-grid img {
    width: 100%;
    height: auto;
    margin: 0 0 10px 0;
}
.related-posts-grid h5 {
    margin: 0;
}
.related-posts-grid h5 a {
    text-decoration: none;
}
</style>