The Infinity Saga left a couple of mysteries and plot holes that the movies in the Avengers: Endgame, which saw all heroes ing forces to defeat Thanos. Saving the universe came at a big price, and Iron Man sacrificed himself to prevent Thanos from wiping out half the universe a second time.
Life in the MCU has to continue, and the first post-Iron Man movie was Mysterio, who presented himself as a superhero from an alternate reality within the Multiverse. Mysterio explained that the snap caused a dimensional tear which allowed him to cross to this universe, but that also meant that the Elementals had crossed too and were ready to destroy this Earth. This turned out to be a lie, and Mysterio was no superhero nor did he come from an alternate universe, and he was a regular human from the same world as Peter.
Beck was actually a former Stark Industries employee who helped develop the holographic technology seen in Doctor Strange in the Multiverse of Madness.
In the MCU, the Multiverse is the collective term for all parallel dimensions in existence, such as the Doctor Strange (where Strange got a glimpse of some of these dimensions), and Avengers: Endgame, as the Quantum Realm was used to make the time heist possible. Those who know about the multiverse is because they’ve explored it one way or another, but Quentin Beck is no superhero nor scientist, and it’s never mentioned how he knew about the multiverse – and he knew enough to come up with the story about an alternate world.
That’s a part of Mysterio’s backstory that needs to be addressed at some point in order to avoid a plot hole, and the best opportunity to do so is in Doctor Strange in the Multiverse of Madness. As the title says, the movie will further explore the multiverse and the dangers of it and will count with the presence of mid-credits scene of Spider-Man: Far From Home, Mysterio is a name that will still be mentioned in the MCU, but his knowledge of the multiverse shouldn’t be forgotten.
