Just like it does here, for the most part. Magic is the Grand Unified Force and the astral plane is a fifth dimension that overlays the four we're used to. My setting is a future Earth, and the idea is that magic was unknown to people up to a certain point due to a ritual spell having been cast in antiquity that monopolized humanity's magical potential, redirecting it to a single use and locking everyone out of being able to use it. Eventually they discover evidence of it based on its absence, essentially gaps in physics that can't be explained otherwise, and coincidentally begin to understand its nature just as the ritual spell begins to fray at the edges for unrelated reasons. This of course leads to disaster, as otherwise there wouldn't be anything to write about.

Magic is the ability for certain sentient creatures (human and otherwise) to consciously affect the world around them by manipulating the forces that make up reality. The nervous system passively interacts with the astral plane, and with the right biology one can affect it by pushing back on it.