I found the solution

When I was doing this $query->select('serial_number'); I was only selecting serial_number and not the column that connects both the modals i.e. product_id inside barcodes table.

$query->select('product_id', 'serial_number');. However this will return 2 columns. If somebody wants just one then he/she will have to use [collection transform][1].

$products = $products->map(function ($product) {
    $product->allBarcodes->transform(function ($q) {
        return $q->serial_number;
    });
    return $product;
});

It would depend on where you want this attribute to be appended. If you want your serial_number attribute whenever you retrieve a model, I would suggest this: https://laravel.com/docs/7.x/eloquent-serialization#appending-values-to-json

In your App\Product.php file, add the following:

    protected $appends = ['serial_number'];

    public function getSerialNumberAttribute()
    {
        return $this->allBarcodes->serial_number;
    }

or if your relationship to barcodes is a One to Many or Many to Many then loop through the collection like this:

    public function getSerialNumberAttribute()
    {
        $result = [];

        foreach($this->allBarcodes as $barcode)
        {
            array_push($result,$barcode->serial_number);
        }

        return result;
    }

All fair options. Also, I know it’s not sexy, but it’s worth remembering you can do DB statements and simply do your joins, etc. I think as Laravel devs we get a bit too attached with Eloquent.