How to get common entities of related model (many-to-many)?

Question

Laravel

Hello! A little stuck, please help,
There is a post, there are tags, a many-to-many relationship;

The question
is how to get all and only those tags that are in all posts
- or get common post tags
- or, for example, make such a selection, all posts with tags (maximum links) and sort in descending order of the number of their links by posts
- or get the intersection of all sets of post tags

pseudocode

Tag::whereHas('posts', function($builder) {
    $builder->count()
        ->sortBy('...')
        ->limit(5);
})
    ->get();

Answer 1

There is no need to do sortBy, get in the subquery.

...get all and only those tags that are in all posts

Tag::whereHas('posts')->get()->sortBy('id'); // Коллекция

Calculate quantity:
Tag::whereHas('posts')->count(); // 100