In the Marvel Cinematic Universe (MCU), the concept of the quantum realm is primarily associated with the Ant-Man movies and explored further in "Ant-Man and the Wasp" and "Doctor Strange in the Multiverse of Madness." While the exact nature and mechanics of the quantum realm within the MCU are fictional and artistic interpretations, it is suggested to be a subatomic dimension that operates on different physical laws.
Regarding the relationship between the quantum realm and the multiverse within the MCU, it's important to note that the specifics of the multiverse and its connection to the quantum realm may vary as the MCU evolves and introduces new storylines. the MCU has indicated the existence of a multiverse with various parallel universes, each with its own distinct reality and set of events.
Within this framework, it's conceivable that there could be a quantum realm associated with each individual universe within the MCU multiverse. Each universe may have its own unique quantum realm that operates independently within its boundaries. However, the exact interplay between the quantum realm and the multiverse in the MCU is subject to the creative decisions of the storytellers and may evolve over time as new narratives and movies are introduced.
It's worth noting that the MCU is a fictional universe, and the portrayal of scientific concepts like the quantum realm and the multiverse is primarily for storytelling purposes rather than an accurate representation of real-world scientific theories or principles.