How do you do that? I don't see a slot that says "Insert $2" anywhere, just a notice that "This video is unavailable."

This person made the mistake of linking directly to the episode. The playlist of the channel [0] allows a logged in person to pay for SD or HD per episode. You can access the playlist of the channel from the page linked by clicking "World's Toughest Fixes" underneath the video title.

[0] https://www.youtube.com/show/SCuCu3O5ATBHFcvv94RsRQWg?season...

All these episodes sound great!

Also available on Disney+ (at least in the UK)

Not on Disney+ in the USA, for some reason. And they HAVE National Geographic in there!