Add Blog related field to REST API

functions.php

<?php
// Add Blog related field to REST API /wp-json/wp/v2/blog

function blog_add_related_posts()
{
    register_rest_field('blog', 'related', [
        'get_callback' => function ($post) {
            $categories = wp_get_post_categories($post['id']);
            $related = get_posts([
                'post_type' => 'blog',
                'category__in' => $categories,
                'posts_per_page' => 3,
                'post__not_in' => [$post['id']]
            ]);
            return $related;
        }
    ]);
}
add_action('rest_api_init', 'blog_add_related_posts');