The intro to the bard links the planes of the multiverse to a theory of magic.
Mechanically, the Bard can choose whether to use the arcane, divine, or primal spell list. This choice implies narratives about magic.
The "Outer Planes" are the source of divine magic, meaning that divine magic is part of the thoughts and dreams, and archetypes, of the Astral Plane. Moreover, the Outer Planes imply this magic is inherently ethical and relates to the alignments.
Oppositely, the Material Plane with its elemental winds and living animals, is the source of primal magic.
One might expect the Ethereal Plane amid both of them to be the source of the arcane magic. It might be to some extent. The arcane source is the immersive "study" of "mystic" knowledge, to make new discoveries about how magic works.
Ultimately, this fluidity with the main three magical sources, leans into the feature of Magical Secrets, and the insight into any form of magic.