Stranger Thing is wrapping up, and before it's end, the Duffer Brothers will answer every mystery of the Hawkings and the Upside Down world.

The world of the Upside Down has hooked billions of people for over nine years, and the answer to what exactly this dark world with vines is has finally been revealed. Dustin (Gaten Matarazzo), the science nerd, reads Dr. Brenner’s (Matthew Modine) journal and discovers that what they thought about the Upside Down is actually something completely different.
So, what has been revealed in Season 5 regarding the Upside Down? Everything is explained below.

In Season 1, the Upside Down shocked the world. The place, covered with living black vines, had creatures like Demogorgons and ash-like particles floating everywhere. While it appeared to be an alternate version of Hawkins, it was dark, gory, and until now, the home of Vecna and all his creatures.

Eleven is the one who opened the gate while using her powers against Henry Creel (Vecna) at Hawkins Lab in 1979. She was also the one who named this place the Upside Down in Season 1. This explains why the primary gate was located in the basement of the lab, where Dr. Brenner was running his secret missions.

In Volume 2 of the final season, it is revealed that the Upside Down is not an alternate version of Hawkins, but rather a “bridge between Earth and space.” The flesh wall serves as the boundary of that bridge.
“It’s a wormhole,” Dustin tells Steve after he discovered Dr Brenner's journals — a “bridge between two points in time and space” connecting Hawkins to yet another world.

Brenner’s notebook reveals a lot about the Upside Down, including an illustration of a giant sphere floating above Hawkins Lab. Dustin quickly comes up with the theory that this shield was generated using dark magic to power the Upside Down’s protection. He compares it to the Death Star II’s protective energy shield from the 1983 film Star Wars: Return of the Jedi.
However, he is eventually proven wrong and later concludes that Vecna did not create it. Instead, it was science that created the sphere, which looked like a fireball.

In Episode 7, Dustin explains what the Upside Down is using a diagram, showing it in a cylindrical shape and how it connects Hawkins to another world, which he calls the Abyss. The group then comes to the conclusion that the Mind Flayer, the Demogorgons, and Vecna live there, not in the Upside Down. Another point that got clear was that Eleven threw Henry into the Abyss, not the Upside Down.