I understand (mostly) linked duplicates from the previous video Duplicating and Instancing, but I do not understand link to collection.
Say I have the default cube in the default collection. I then made a new collection called Linked. I linked the cube in the default collection to the linked collection. I now see the cube in both collections, and see object properties has 2 users, but there is still only one cube to manipulate?
Yes it is always one cube, just that it is located in two collections. It's just like having one kid and two rooms, he can be in one room (collection one) or he can go to the other room (collection two). Just that in Blender it happens that the kid can be in room one and two at the same time. Your cube can live in both collections at once.
This is handy when you have to do some scene organization and you need one asset to be in more than one collection because you have to keep turning one collection on and off but you don't want that object to be hidden, or something like that. There's lots of situations.
Better to think of it in terms of "Membership" in a "Set". An object can be a "member" of more than one information set.
To use the kid analogy from above, one kid (object) can be a member of three clubs (Blender Collections).
His name appears on the roll call of each club, but he is still only one kid.
As Omar said, once you grasp that, it's a very useful and powerful concept.
Thanks to all for your terrific responses - it's much clearer to me now, and I see how it can be very useful!