Once an order is completed, any custom fields will be attached to the order and visible in the Shopify admin.
Where the custom field is attached will depend on what type of field it is.
Both line item and order fields will have a key ( Grind, Membership ) and a value ( Beans, Q154D3245 ). If you are looking to use the custom fields in your theme, with Shopify Flow or in any other integrations, the key can be used to detect each individual custom field.