Hi:
We'll ask the course instructors to respond to this question.
Regarding page 3-23, they started with the AMESHOUSING3 data to make the INTERACT data used by the rest of the analysis:

Regarding the data, on page 3-23 it appears that they are using the AMESHOUSING3 data from 3-18 to make the dataset INTERACT, as shown below:

(sorry, I posted the wrong screen shot before)
Thanks, in advance, for your patience,
Cynthia