I added a "how did you hear about us?" form to my checkout, and the Shopify tutorial says I can find customer's answers under additional details, but I don't see this anywhere on my orders? Where is the data captured?
The Shopify admin is designed to display a special section for 'additional details' on the order details page if an order contains cart attribute data. When looking at an order in the Shopify admin, you can find the "Additional details" section right underneath the section for "Notes". See the following screenshot for an example:
Hopefully this helps!