What is the trick to kneading silk smooth bread? I've been teaching myself how to make bread for a few weeks, had some success but never able to create dough that is completely smooth why? 

Following a recipe like this one 

https://www.youtube.com/watch?v=hTDvqn5SLGs



You can see that the bread is literally like plastic. 

But when I knead my dough (even with a mixer machine) the dough is more like this:



Recipe shown in video (and what I used in my attempt) is 




300g Medium-fat dough
160g milk
3g instant dry yeast
30g sugar
6g veg oil
"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.