If you are working with unknown data, you will probably need to look at using a SAX style JSON parser that can trigger events as it parses. The Jackson Core Java library offers this, but you’ll probably have to write your own Java Action in Mendix to do this, and have it trigger a callback to a microflow for each event.
If the data structure is known, look at using a JSON Structure and creating and Import Mapping to map it to a Mendix entity. You will then be able to query the entity directly.
1: Are you dealing with a JSON file or with Mendix objects in a list?
2: before jumping into this; please follow the learning path https://learn.mendix.com/link/path/38
I guess my real issue is this: Why are we able to RETRIEVE the list of lineItems. but I am unable to actually put that list into a lineItems structure.