This article contains spoilers for Stranger Things season 4 volume 1.
the Mind-Flayer and the Demogorgon. The origin of the Upside Down has long been something of a mystery - but, given Stranger Things season 4 is marketed as "the beginning of the end," it's no surprise the Duffer brothers have decided now is the right time to provide some answers.
An important clue to the Upside Down's origin was provided in the Stranger Things: Kamchatka miniseries by Michael Moreci, Todor Hristov, Dan Jackson, and Nate Piekos, released ahead of season 4 as an official tie-in. This introduces readers to Russia's version of Dr. Brenner, Dr. Boris Orlov, whose theories appear to be directly related to the creation of the Upside Down. Like Brenner, he has been conducting experiments in telekinesis; significantly, he believes this power can serve to open doorways to "infinite worlds." It seems is embracing the multiverse.
This fits perfectly with Stranger Things season 4, episode 7, which finally reveals Eleven's true origin story. It seems the first "window" to the Upside Down was created in the aftermath of a telekinetic battle between Eleven and Number One, with Eleven tearing open one of Dr. Orlov's "doorways" - and blasting her opponent into it. Another connection was forged some years later, when Eleven opened the first Gate - bringing the two dimensions more closely together than ever before, with a rough copy of Hawkins at the moment of the Gate's creation overlaid on the existing dimension. This would explain why the Hawkins of the Upside Down is apparently frozen in time; it was copied at the precise moment Eleven tore open the Gate.
All this means Eleven did not create the Upside Down. This Hellish dimension already existed, ruled over by the hive mind of the Mind-Flayer. She did, however, transform it - drawing the different planes of existence closer together, and placing a copy of Hawkins upon the Upside Down like some kind of overlay. The barriers between the dimensions are presumably still dangerously thin, explaining why Number One is able to open new Gates from the other side.
This may well hint at Number One's plan in Stranger Things season 4's present day. The Gates he is opening may well serve to draw the dimensions even closer together, and if enough Gates are opened, it's possible the two planes will be bound together permanently, with the Upside Down merging with Hawkins. Indeed, the earthquakes shaking the Upside Down could be hints of dimensional instability, with the Upside Down more profoundly affected because it seems to be a smaller dimension. This would explain why the Mind-Flayer isn't taking advantage of the Fates to launch an invasion; it is conserving its forces for the merging. If this is indeed the case, Stranger Things season 4 volume 2 will see matters escalate even more.