I see what you want to do.
The product custom fields allow you to add information to your product via the back end and display it on the product page. It's used for instance for specifications of computer hardware. Later on, users will be able to search through those fields via a special search module.
The goal wasn't to allow customers to enter information via the front end.
You can have dropdowns so that your users select them on the product page with the characteristics/variants feature we provide. With this feature, you set the values of the characteristics, attach the characteristics to your products and the system generates variants of the product based on them. Then, the customers can select the options they want.
However, the customer can't enter any info themselves and it's not something we accounted for with this. The only possibility for now would be to add custom address fields which are for the user to enter.
Could you explain a bit more the purpose of those fields ? The product information is usually entered by the store, not by the customers, so I'm not sure what you want to achieve. Do you really need those fields on a per-product basis in the order or would it be ok to have one instance of those fields for an entire order ?