1) At 5:00, What is this structure and can players place their own trade workshops/ specialty workbenches on them?
These are plazas that were introduced with the 1.0 patch. They are the closest thing to player created towns AA has. They are player made and very expensive. They come with vendors, bank, bind points AND their own trade cart with unique mats. In order to use the trade cart, you need to own property in the zone which the plaza is in. Every zone has a different trade cart tied to their plaza with unique mats.
2) At 24:07, Is this some kind of stable that you can place cows or animals in?'
Look at these as the mega seeds for animals. 5x of an animal type+lifepoint item will allow you to make these assuming you meet the farming skill (starts at 10k). They are permanent structures and they allow you to obtain a higher yield than a single animal. Whether that be harvesting wool, feathers, milk, etc or mass killing them all at once (structure is destroyed). Another benefit of these is that they do not need feeding to keep it up, so you don't have to feed them every day to keep them alive but rather when you want to harvest from them. They do get sick occasionally.
3) At 28:22, I was unaware that Koreans had a portable Auction House. If so, is this something that comes from a cash shop item, or is it just standard?
The AH/Bank/vendor has been in KR for a while and is a player created item not a cash shop item. Also they are not portable but rather for housing.
Hope this clears things up.