The Simplest Way to Add Related Posts to Your WordPress Site Without Plugins

$related = new WP_Query(
        array(
                'post_type'      => 'tutorials', //YOUR CPT SLUG
                'category__in'   => wp_get_post_categories( $post->ID ),
                'posts_per_page' => 3,
                'post__not_in'   => array( $post->ID ),
        )
);

if ( $related->have_posts() ) { ?>
<h3><?php _e( 'You might like this too!', 'hoolite' ); ?></h3>
	<div class="cpt-list">
		<?php
		while ( $related->have_posts() ) {
			$related->the_post();
			?>
			<div class="cpt-post">
				<div class="cpt-post--wrapper">
					<?php
					$featured_image = get_the_post_thumbnail_url(get_the_ID(),'medium'); // THUMBNAIL, MEDIUM, LARGE, FULL.
					$default_image = get_site_url() .'/wp-content/uploads/2022/02/my_backup_image.jpg'; // URL TO YOUR DEFAULT/BACKUP IMAGE.
					$post_thumbnail = has_post_thumbnail( $post->ID ) ? $featured_image : $default_image; // CHECK IF FEATURED IMAGE ELSE USE DEFAULT IMAGE ?>
					
					<a href="<?php echo esc_url( get_the_permalink() ); ?>" title="<?php echo esc_html( the_title() ); ?>">
						<div class="cpt-post--thumb" style="background-image: url('<?php echo $post_thumbnail ?>')"></div>
					</a>
					<div class="cpt-post--text-inner">
						<a href="<?php echo esc_url( get_the_permalink() ); ?>" title="<?php echo esc_html( the_title() ); ?>"><h3><?php echo esc_html( get_the_title() ); ?></h3></a>
						<?php echo wp_trim_words( get_the_content(), 25 ); ?>
					</div>
				</div>
			</div>
			<?php
		}
		wp_reset_postdata();
		?>
	</div>
	<?php
	} else {
		esc_html_e( 'Woops! Nothing found!', 'hoolite' );
	}

/* EXAMPLE CSS */
.cpt-list {
    display: flex;
    flex-wrap: wrap;
    margin: 0 -16px;
}
.cpt-list .cpt-post {
    padding: 0 16px;
    margin-bottom: 32px;
    width: 33.33%;
}
.cpt-list .cpt-post .cpt-post--text-inner {
    padding: 16px;
}
.cpt-list .cpt-post .cpt-post--thumb {
    height: 200px;
    background-size: cover;
    background-position: center;
}
.cpt-list .cpt-post .cpt-post--wrapper{
	box-shadow: 4px 8px 40px 0 rgb(0 0 0 / 10%);
	height: 100%;
}