Short answer: Will Byers was taken into the Upside Down by a Demogorgon in the first season, likely after fleeing a Demogorgon with his bike ride home, and he survived there for about a week by hiding, using the environment, and trying to communicate with his mother; later developments tease Vecna’s deeper involvement and Will’s unique connection. Details and context

• Abduction and entry
  • In the first season, Will Byers is pursued by a Demogorgon while riding home from Mike’s house. He flees and is pulled into the parallel dimension, which results in his disappearance from the town of Hawkins and marks the onset of the Upside Down storyline. His entry is tied to the Demogorgon’s pursuit and the Gate-like openings that briefly appear in the world. This is established through Will’s disappearance and the early clues in the series.

• Survival in the Upside Down
  • Will survives for roughly a week by hiding in familiar spots such as the shed and Castle Byers, and by avoiding direct contact with the Demogorgon. He remains largely hidden while the creature searches for him, leading to a tense period of uncertainty for his friends and family. His ability to signal to his mother via the home’s lights becomes a notable moment of attempted communication. This survival arc is reinforced by multiple retellings and supplementary materials, including comics that fill in some gaps.

• Rescue and aftermath
  • Joyce Byers, Hopper, and others eventually locate Will by entering the Upside Down through a gate connected to Eleven’s powers. Will is physically rescued, though he bears a literal emblem of his ordeal—a tendril-like intrusion in his throat that is removed during rescue. The experience leaves lasting effects, including his ongoing connection to the Mind Flayer and a heightened awareness of danger in the Upside Down.

• Ongoing mystery and season 5 hints
  • The long-term questions about Will’s status and his exact role in relation to Vecna and the Upside Down have been explored in various media pieces and fan discussions. Newer materials and trailers tease deeper revelations about Vecna’s influence and Will’s significance in future storylines, suggesting a more complex purpose behind his original kidnapping and survival.

If you’d like, I can pull together a concise timeline of the key moments from Season 1 through Season 5 that pertain to Will’s journey to and within the Upside Down, with paragraph-by-paragraph citations to the sources above.