PHP Laravel：理解这个闭包

I have this snippet from the Laravel documentation:

DB::table('users')
            ->whereExists(function ($query) {
                $query->select(DB::raw(1))
                      ->from('orders')
                      ->whereRaw('orders.user_id = users.id');
            })
            ->get();

I need to understand two things.

1. Where does the $query parameter to the closure come from? I suspect that there is something happening under the hood that I don't understand. The function takes 1 parameter, the $query, but where does it come from, how does this function know what is in this parameter, what is passed into the function?
2. It appears that this closure doesn't return a value, there is no return statement. So how does the whereExists method know the return value of the closure?

Refer to the source: https://github.com/laravel/framework/blob/987a21f39f203c76665f6014cbef10451689fbdd/src/Illuminate/Database/Query/Builder.php#L1333

As you can see the closure is treated like a callback.

So whereExists passes $query to it. $query is an instance of self (Builder) class, so the code in the closure is just updating the object.

/**
 * Add an exists clause to the query.
 *
 * @param  \Closure $callback
 * @param  string   $boolean
 * @param  bool     $not
 * @return $this
 */
public function whereExists(Closure $callback, $boolean = 'and', $not = false)
{
    $query = $this->forSubQuery();
    // Similar to the sub-select clause, we will create a new query instance so
    // the developer may cleanly specify the entire exists query and we will
    // compile the whole thing in the grammar and insert it into the SQL.
    call_user_func($callback, $query);
    return $this->addWhereExistsQuery($query, $boolean, $not);
}