Related posts at the last position in the content.

functions.php

<?php
/**
 * Add related posts at the last position in the content.
 * URL to Plugin: https://de.wordpress.org/plugins/wordpress-23-related-posts-plugin/
 *
 * @param string $content post content.
 * 
 * @return string
 */
function my_related_posts( $content ) {
	
	$post_types = [ 'post' ];

	if ( ! function_exists( 'wp_related_posts' ) ) {
		return $content;
	}
	
	if ( ! is_singular( $post_types ) ) {
		return $content;
	}

	ob_start();
	wp_related_posts();
	$content .= ob_get_contents();
	ob_end_clean();

	return $content;
}

/* Apply filter for related posts */
add_filter( 'the_content', 'my_related_posts', PHP_INT_MAX );