The Russians wanted to open a gate to the Upside Down (an alternate dimension in Stranger Things) to gain a Cold War advantage, hoping to weaponize its monsters (like Demogorgons) or use it as a secret, undetectable portal for military infiltration into the United States, bypassing defenses. They realized Hawkins, Indiana, was a weak point in spacetime due to a previous gate there, making it easier to open one under the Starcourt Mall.
The Russians want access to the Upside Down probably to gain a military advantage over the US. Think of the Upside Down as an analogy to nuclear weapons. At the beginning of S1, we see that the US government is conducting experiments to access it (well, they were experimenting on Eleven.
The Soviets occasionally executed prisoners by feeding them to the creature, helping it grow stronger in the process. At some point, the Soviets also captured a large number of younger, adolescent Demogorgons, which were paralysed and kept in tanks filled with oxygenized water.
Second, the Russia storyline keeps the show grounded in real-world 1980s fear. The Upside Down is supernatural Cold War anxiety; the Soviets are the real one. That tension was baked into the show from season one. Without Russia, Season 4 would be wall-to-wall cosmic horror with no human counterweight.
When Eleven was trying to find the Russian guy she accidentally encountered the Demogorgon and was scared by it. As season 2 tells us extreme emotions (such as anger or in this case fear) make Eleven more powerful. This is how the gate was opened.
After it was discovered that American scientists had accidentally opened a gate to an extra-dimensional realm in late 1983, the Soviet Union grew intent on creating their own gate, hoping that access to the dimension could give them an advantage in the Cold War.
Yes, the Demogorgon did essentially "impregnate" Will in Stranger Things Season 1 by implanting a slug-like larva into him, using his body as a host to grow and produce more creatures for the hive mind, a process that was later revealed to be part of Vecna's larger plan. Will coughed up this larva, which then hatched into a polywog (stage two of the Demogorgon life cycle), demonstrating how the Upside Down creatures reproduce and spread, as seen with Barb Holland as well.
The Russians had a Demogorgon even after the Hawkins gate closed because they captured one through the larger, more stable gate beneath the Starcourt Mall before it was sealed, or through a separate, failed attempt at opening their own gate in Russia.
While there, Alexei explained that the Soviets had specifically created a base in Hawkins because it was previously a site of an interdimensional incursion, and thus was a weak point in the fabric of space-time that could be exploited by the Key.
Yes, Dart (D'Artagnan) is a baby Demogorgon, specifically an adolescent stage called a "Demodog," which Dustin Henderson found and raised in Stranger Things Season 2; he's essentially a younger, less aggressive version of the main monsters from Season 1, evolving from a pollywog-like creature into a four-legged, predatory form.
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The victim's suffering, panic and eventual death seemed to be key to Vecna's ability to open gates, likely maximizing the strength of the psychic connection. Vecna's ultimate goal was to open four gates. Vecna's first known target was Chrissy Cunningham, a popular student and head cheerleader at Hawkins High School.
It was a sixth-stage Demogorgon that was captured by the Soviets in the Kamchatka gulag, where it was used to murder prisoners.
008, or Kali Prasad, is Eleven's "sister" from Hawkins Lab, introduced in Stranger Things Season 2 to expand the world of other numbered children and challenge Eleven's moral compass. Her point is to show Eleven a different path—using her illusion powers for revenge and vengeance rather than pure heroism—forcing El to choose between Kali's dark path and returning to help her friends in Hawkins, ultimately strengthening El's resolve and deepening her understanding of her own identity and purpose.
This event occurred when Eleven, a child test subject from Hawkins National Laboratory with psychokinetic abilities, made transdimensional contact with the Demogorgon. This contact inadvertently opened a gate between dimensions at the lab, allowing the Demogorgon to set foot in Hawkins and begin its reign of terror.
As for why they would want to open the gate at all, remember that this is during the Cold War. Both sides are terrified of falling behind the other. If the Americans have opened a gate to another dimension, then the Russians want to do so too.
Scratchmen Apoo is an informant and member of the Beasts Pirates. He was previously part of the On Air Pirates and attempted an alliance with the Kid and Hawkins Pirates. However, he betrayed them to Kaidou, the Beasts Pirates' leader, resulting in Kid's defeat and Hawkins' forced alliance with Kaidou.
"So, Hawkins, Indiana, isn't real, but the lab, the experiments, the monster, that story started right here," said Magical Hijinx, concluding her story. In fact, tourists can visit Camp Hero State Park, Montauk, New York.
Eventually, he came under the possession of the Mind Flayer, who used him as a vessel and as a tool to gather the civilians of Hawkins, to create a larger physical vessel to host the Mind Flayer: the Spider Monster.
David Harbour (Jim Hopper) has stated that the dog was one of the worst "actors" to work with, even making him storm off set on one occasion. This most likely resulted in Chester being retired from the show.
In the only cases of Demogorgon reproduction observed, the cycle began with the deposition of eggs or larvae within a host by a tendril within the Upside Down. The slug-like larvae were later vomited-up by the host, before subsequently escaping into the wild.
In Stranger Things, several characters say the F-word, including Will Byers, Billy Hargrove, and most notably, Joyce Byers (Winona Ryder) in a significant Season 4 moment when confronting Vecna, delivering a rare and impactful "You f***ed with the wrong family". Other instances come from characters like Billy (often possessed), and even faint or muffled uses have been noted from others like Jonathan.
So, in conclusion, the Mind Flayer possesses the Demogorgon, whom was 001, proving that the Mind Flayer cannot be 001. This explains how Brenner survived, how the Demodogs were created, and how and why the other test subjects of Brenner's escaped.
Mews was killed and partially eaten by D'Artagnan who ate her body in order to grow larger. Her body was discovered by Dustin. Dustin hid Mews's death from his mother and sent her to search for the cat in another neighborhood, giving her hope that she was still alive.