It seems like your query is wrong, as far I can understand you want join two tables which will make your query as

SELECT products.name 
FROM products 
    JOIN subcategories on products.subcategory_id = subcategories.id 
where subcategories.name=Extensions

which can be written with Laravel Eloquent as:

Product::join('subcategories', 'products.subcategoryid', '=', 'subcategoories.id')
        ->where('subcategories.name','Extensions')
        ->selectRaw('products.name as name')
        ->get()
        ->pluck('name')

A better explanation of your requirement will help to answer better.