One of the biggest questions surrounding Stranger Things is how Will Byers (Noah Schnapp) managed to survive while being stuck in the Upside Down. In season 1, Will was trapped in the dark dimension. The effects of his stay in the Upside Down went on to have even more serious consequences in season 2.
The latest season of Stranger Things revealed that Eleven was forced into experimenting with her telekinetic powers at Hawkins National Laboratory. She defeated Vecna but opened a rift between parallel dimensions. The alternate dimension mirrored the human world but lacked light and the air was toxic.
In the first episode of Stranger Things season 1, Will was taken into the Upside Down by the Demogorgon. We don't know the exact details surrounding his abduction as they were never clarified in the Netflix series but the Stranger Things comic miniseries published by Dark Horse Comics provided a lot of info. While attempting to evade Demogorgon, Will hid in his backyard shed. He shot at the Demogorgon with a rifle but was still transported to the parallel dimension.
Will spent almost a week in the Upside Down and managed to stay alive. After his abduction, Will tried to communicate with his family and friends. Her mother tried her best to save his son which presumably gave him the strength to survive. That may have been true, but the comic book series gave more insight into how Will survived.
The Upside Down reminded Will of a Dungeons & Dragons campaign. His character in D&D Campaign was "Will the Wise" and he bravely fought the Demogorgon by unloading all the ammo in the hunting rifle. This scared the monster off, giving Will time to explore the Upside Down. He was an expert at hiding from the Demogorgon on various occasions. Will tried to help others, but he was too late when it came to Barb. But he saved Nancy from the Demogorgon when she found herself in the Upside Down. After having no way out, Will took shelter in Castle Byers. As he grew weaker, the Demogorgon abducted Will. His body was taken over by vines which might have been keeping him alive for the Demogorgon to feed on later, but he was soon rescued by Joyce and Hopper.
Though we don't know how Will survived in the Upside Down without food and water. The comics revealed that Will began feeling weaker the longer he spent in the parallel universe. After the release of season 4, we have got a lot of new information about the Upside Down, meaning that fans have come up with new theories about how Will could've survived. One of them is that Vecna Intentionally kept Will alive In the Upside Down.