Those code parts are the final product of the according tutorial series for unity.
Download and unzip the package. The package contains two projects. The "initial project" shown at the beginning of the video and the final results from the end. Inside the Unity Hub select "Add" and select the project folder you want to import.
Create the basic setup. You will be able to draw single items on click onto a tilemap.
Expand the drawing part by rectangles and lines.
Draw onto different tilemaps based on the categories you are using. This allows stacking of items on a floor.
Automatically create the UI based on UICategories and all existing (and assigned) buildables.