It's actually very discoverable. The video lacked the sidebar showing the keypresses, though.

Generally, each time you press a key that is part of a sequence, you get a pop-up explaining what further keys are available. So for instance, I opened magit (M-x magit-status) on a project and pressed ? (for help):

https://i.imgur.com/c5cmmpB.png

The popup lists the keys I can use on magit-status page, and if you know git it's pretty obvious where to go in most cases.

Also, at any time you can press ? and then one of the key listed in the popup to get a detailed description and/or an appropriate manpage.