At 7:17 you selected the all side block with one click! how did you do that!? with "A" I select the whole wall. With shift+alt click I get only some faces selected...
The "L" hotkey triggers a 'select linked' operator. So if you select one face and hit CTRL + L it will select all other faces that are connected to the selected face. Alternatively you can hover over a face and hit "L" and it will select all connected. Inversely SHIFT + L will deselect all connected faces.