к
$posts->map(function ($categories,$users){
$this->update([
'category_id' => $categories->random()->id ,
'user_id' =>$users->random()->id,
]);
});
вариант без map-----
foreach ( $posts as $post) {
$post->update([
'category_id' => $categories->random()->id ,
'user_id' =>$users->random()->id,
]);
}