How to get Shipment item collection by shipment item id Magento 2?

You can retrieve the shipment item data by shipment item id using Magento 2 by Shipment Item RepositoryInterface.

Used Shipment item repository interface to get shipment data:
Magento\Sales\Api\ShipmentItemRepositoryInterface

An interface contains the get(int $id) method to fetch record by shipment id.

using the above repository, You need to pass shipment item id from the sales_shipment_item table.

Please be careful of shipment_item_id will be used and not the shipment_id from the shipment table.

<?php
namespace Jesadiya\ShipItem\Model;

use Exception;
use Magento\Sales\Api\Data\ShipmentItemInterface;
use Magento\Sales\Api\ShipmentItemRepositoryInterface;
use Psr\Log\LoggerInterface;

class Demo
{
    /**
     * @var LoggerInterface
     */
    private $logger;

    /**
     * @var ShipmentItemRepositoryInterface
     */
    private $shipmentItemRepository;

    public function __construct(
        LoggerInterface $logger,
        ShipmentItemRepositoryInterface $shipmentItem
    ) {
        $this->logger = $logger;
        $this->shipmentItemRepository = $shipmentItemRepository;
    }

    /**
     * @return ShipmentItemInterface[]|null
     */
    public function getShipmentItem(int $shipmentItemId): ?array
    {
        $shipmentItems = null;
        try {
            $shipmentItems = $this->shipmentItemRepository->get($shipmentItemId);
        } catch (Exception $exception)  {
            $this->logger->critical($exception->getMessage());
        }
        return $shipmentItems;
    }

Pass shipment item id as arguments in the method getShipmentItem() and you can fetch the shipment item as a result.

$shipmentItemId = 100;
$shipmentData = $this->getShipmentItem($shipmentItemId);
var_dump($shipmentData->getData());

The output will be from the sales_shipment_item table,

array (size=12)
  'entity_id' => string '100' (length=3)
  'parent_id' => string '211' (length=3)
  'row_total' => null
  'price' => string '270.0000' (length=8)
  'weight' => string '100.0000' (length=8)
  'qty' => string '1.0000' (length=6)
  'product_id' => string '110' (length=3)
  'order_item_id' => string '1121' (length=4)
  'additional_data' => null
  'description' => null
  'name' => string 'test product name' (length=17)
  'sku' => string 'test1234' (length=8)

You can get the shipment item record from the item id in Magento 2 with the simple code snippet.

From the above result, you can fetch required data as per your requirement.