When an order has been placed with your customized products, these customizations will be added as line items for every customized product. You'll be able to view them in your Shopify admin by clicking on "Orders", then clicking on the order number:

If your customized product contains a file upload as part of the order, then a link to the file will be included, which will allow you to download the attached file.

You can add customizations to confirmation emails as well if you'd like to view them elsewhere.