Something they still haven't explained... (spoilers)

Okay, we get that the Upside Down was originally a formless alternate dimension that Eleven banished Henry to. We get that Henry (later renamed "Vecna,") used his abilities to shape it and the creatures within.

What they STILL have not explained is how the Upside Down went from a strange, rocky, cold, nearly lifeless landscape with stormy clouds, toxic fumes, and monsters; to an exact copy of Hawkins in 1983, and why it didn't change over a period of 3 years.

I mean, if Henry had wanted to recreate Hawkins in general, he would have recreated the town from when he knew it in the late 40s, because after his family got offed and dad went to prison, he spent the next 20 years in laboratories, indoors and had no idea what the place looked like. But then again, they almost never show him outdoors as a child, so it's possible he never got a chance to see that the town was like post-WWII.

Some people have speculated that he sensed Will's mind when the Demigorgon kidnapped him, and he used Will's memories of the town to rebuild it, albeit a twisted, creepy, nearly lifeless version covered in tentacle vines.

But still, it hasn't been confirmed and like I said, we don't know why Vecna re-created Hawkins, and he never explained it to El during their confrontation in his mind.