Dedicated Query String Filtering

I am attempting to create a filter search function from https://laracasts.com/series/eloquent-techniques/episodes/4. I am able to filter names, but I have difficulties in querying many to many relationship here as it uses 'builder' and 'class'.

how do I filter my 'maincategories' here:

public function name($search) // company?name
    {
        return $this->builder->where('company_name', 'LIKE', "%$search%")->orWhere('company_description', 'LIKE', "%$search%");
    }

    public function maincategory(Request $request)
    {
        $companyMainCat = $request->get('main_cat_name');

        return $this->builder->whereHas('maincategories', function ($query) {
            $query->where('main_cat_name', $companyMainCat);
        }); // here's my attempt
    }