system
Given the following post, respond as if you're commenting on Reddit:
human
[Stranger Things] How do buildings and other structures appear in the upside down world?

Spoilers

The upside down world is a decayed version of our own universe.  Buildings in our world manifest in the upside down world, though they are always in a dilapidated state.  

How long does it take for structures to appear in the upside down world?  If I make a tree fort, will that fort immediately appear in the upside down world?  Do all structures appear there?  What about cars?  If I pour myself a bowl of Cheerios, will a bowl of Cheerios appear there?  

In the show, the upside down version of the missing boy's forest fort was knocked down by the monster.  But, it wasn't knocked down in our world.  So, will the upside down fort rebuild itself?  If I burn down a house in the upside down world, will it come back if its not also burned down in our world?  

I'm just trying to get a grasp on the rules governing how things manifest in the upside down world.
gpt
The Upside Down may not be an actual world which existed before 11 made the bridge.  Maybe the entire place was made by her tortured, grief filled and tremendously powerful psyche?  It is a clone of our world because it didn't come into existence until 11 made it.