This snapshot of order detail page. I Just want to known that which file or function call for setting this collection in this page. I want to just get Selected (Filter Row) Items Order.
Check the assigned block to render the order items defined in layout file
<block class="Magento\Sales\Block\Adminhtml\Order\View\Items" name="order_items" template="Magento_Sales::order/view/items.phtml">
Block method \Magento\Sales\Block\Adminhtml\Order\View\Items::getItemsCollection() responsible for serving order item data to this page template.
You can get an idea to filter order item data on that collection in order model:
\Magento\Sales\Model\Sales::getItemsCollection($filterByTypes = , $nonChildrenOnly = false);
Is the method for getting items for section,If you want to MOdify that item collection then you have to work on that method