My students and I are going thru CS Discoveries Unit 4 for the first time. Unlike Unit 3 there are little to no true examples of how to implement any functionality of the ui elements. The unit seems to stop short of adding real functionality. It appears that a full version of App Lab might offer more but there is no obvious way to take my student projects and bring them into App Lab (full version) to further develop what they started.
The goals of Unit 4 include “consider and understand the needs of others while developing a solution to a problem” as well as “prototype solutions both on paper and in App Lab” (from the CSD curriculum guide). So, Unit 4 introduces students to App Lab on a very introductory level with the intent of creating a digital prototype of their app and not a fully functioning app. Student go deeper into App Lab in Unit 6 and even deeper if they continue with computer science and take Computer Science Principals, usually taught in high school. I hope this clarifies the introductory nature of Unit 4.