About cutting edge loop and Alt-LMB HOTKEY

Hi Kent, i would like to thank you for your video.

I have bunch of question to ask you, please help me to figure it out.

Firstly, as in the picture below. I dont understand why you mentioned about "cutting edge loops for the bottom". At the bottom, i remember that we didn't use Alt-LMB for doing this thing.

Secondly, I don't really understand what alt-lmb stands for in this situation. And is it different from using L as we used at the side planks of the box.

Thanks for your help!