Am I correct that you’re meaning products as in WooCommerce products?
If so:
Right before WooCommerce returns the related products for a current product(if you’re not on a product page you could define a post_id yourself). It uses a filter which you can hook into.
$related_posts = apply_filters( 'woocommerce_related_products', $related_posts, $product_id, array(
'limit' => $limit,
'excluded_ids' => $exclude_ids,
) );
By using the following filter within your child-theme/theme:
add_filter( 'woocommerce_related_products, function( $related_posts ) {
// Add or replace products to $related_posts.
return $related_posts;
}, 10, 1 );