This question is attracting close-votes. It's about development, but asked from a user point of view. I'm not voting because I don't have a good feel for whether "how do developers do it" questions are interesting to non-developers, and I'd like to hear people's thoughts.

@DanHulme I'm the close-voter. I've watched the video, and its about designing (prototyping) the UI layout only. There's no hint of making the prototype magically useful in development, it's basically going back to writing code 101. If I have to answer that question, I have to explain about XML layout files, event listener and how it interacts, etc, which I don't see it interesting for general Android user.

@AndrewT. I didn't watch the video, but yeah, that matches my assumption of what it was about. TBH, I imagined an answer to the question wouldn't explain all those things, it would just explain that they are necessary without going into details about what they are.

essentially, explaining that the picture in Illustrator is just a picture, the same way an architect's sketch is just a picture to help imagine what the building would look like