High Stats During Game

So this is something that I've been wondering for quite some time. I see a lot of youtubers (SoloRenektonOnly, Anklespankin, Redmercy, etc...) get champs to nearly 1-2k AP/AD and then just cheese the entire game. How are they able to do that given the items they choose. I don't think they build multiple of the same item, instead they build different items...how are they able to do that without ever trying? Is it even a good thing to build 2+ of the same item in like Unranked or ARAM?