how to write multiple arrays of objects into json file using php?

Because you're writing to the file inside the loop, you keep overwriting its contents every time.

To write all the data to a file, and make it write a valid JSON entity which will be decodable again later, you need to construct a single array containing all your product data, and then encode and write that array to the file once, after the loop has ended.

For example:

$products = array();

foreach($crawler as $node)
{
    $title = $node->filter('h3')->text();
    $img = $node->filter('img')->attr('src');
    $color = $node->filter('div.my-4 div.flex div.px-2 span')->attr('data-colour');
    $capacity = $node->filter('span.product-capacity')->text();
    $availibity = $node->filter('div.text-sm')->text();
    $shippingText = $node->filter('div.bg-white > div')->last()->text();
    $shippingDate = $node->filter('div.bg-white > div')->last()->text();
    
    $productArray = array (
          'title' => $title,
          'price' => 12,
          'imageUrl'=> 'https://www.magpiehq.com/developer-challenge/smartphones/'.$img,
          'capacityMB' => $capacity,
          'colour' => $color,
          'availabilityText' => $availibity,
          'shippingText' =>$shippingText,
          'shippingDate' =>$shippingDate
    );
    
    $products[] = $productArray; //add the current item to the overall array
}

//encode all the data at once, and then write it to the file
$json = json_encode($products);
file_put_contents("output.json", $json);