I have seen tutorials where the instructor accessed the villager's inventory, the one where you can load items in and even put armor on the villager just like the player's character inventory. There is a store here in the Empire where villagers have all kinds of hats and clothing. I can't seem to find any info about it other than the fact that villagers have it. So how do you access this inventory?
Uh...I've never heard of villagers having an inventory that you can access besides the trade menu. Villagers have an invisible inventory that they use to store crops to give to each other. I believe you can put something, like a player or mob head, on a villager's head with a dispenser, but that's it. I think the only way to put armor on a villager is to use commands or mods. Even when the villager is wearing armor, it wouldn't be visible (same as with most mobs). The video you watched must have used mods because villagers have no player like inventory in vanilla.
There is currently a store here on SMP9 that the villagers are waring different clothing and different heads. That player had to be able to access that hidden inventory to do that.