What is the trick to kneading silk smooth bread?
"Cloud buns" and your bread dough are really two different products.  The dough in the video is likely based on a "milk bread" recipe, which results in the smooth texture that is pictured.  It reminds me of the Momofuku Pork Bun, a recipe I have made often.  This recipe produces a very smooth dough, which is portioned, shaped, and steamed.  It has a much different texture than my rustic sourdough loaves, which are generally a higher hydration than the dough you've pictured, but  not nearly as shiny and smooth as the milk bread.  It's just the nature of the recipe and the ingredients.  You can knead all you want, but you are not going to get the same texture and sheen, likely because your bread dough does not include milk and fat.