The magic arises from the collective unconscious of every living thing on Earth (including plants and micro-organisms), manipulated by superhumans with the correct genetic predisposition. However, all superhuman powers do not arise from 'magic'. The origin of their powers allows Mages powers far more wide ranging and versatile than those of 'normal' superhumans and allows the achieving of 'impossible' effects by the collective unconsciousnesses deeper link to the structure of reality. The mage has to believe in what they are doing, so they think it is correct, and it will work. It feels like they are tapping a huge 'sea' of power; precisely how this power manifests, and how it is controlled, depends on the beliefs of the mage. If a Mage doubts his beliefs he could lose part or all of his powers. Mages who are mad and completely detached from the real world, so their belief systems bear very little resemblance to reality can be very powerful and capable of very strange effects.
All living things contribute to the unconscious, but the more intelligent life-forms contribute more. The collective unconscious is effectively a "worldsoul" and could be considered to be Gaea [there could also be an even larger universal "soul"] although it is totally non-sentient. Dreams occur there and partly arise from it. It has no "landscape" but changes as the contributions to it change. Demons, angels etc. can be summoned but are really constructs 'built' to the Mages expectations when they are summoned.
The difference between magic and other superpowers is that superpowers are 'internal' to a person, whereas magic arises from manipulation of the 'worldsoul' and a persons beliefs.
Between the "real" world and the collective unconscious is an intermediate "layer", the spirit plane. This is reachable by Extra-Dimensional Movement (with "Leaves Body Behind", -1) and can be seen into with Spirit Sight (Sense the Spirit Plane, Ranged, Discriminatory; or Clairsentience into the Spirit Plane). Mental Awareness can also give glimpses into the Spirit Plane (some animals also possess this ability, making them react strangely under certain circumstances). The collective unconscious cannot be reached by the same type of Extra-Dimensional Movement used to reach the Spirit plane, but glimpses can be seen from the Spirit plane with Spirit Sight. It is an 'interpretative' place based on how things see the real world mixed with interpretation through the unconscious. Passage to the Spirit Plane is similar to those experiencing near-death - moving along a dark tunnel, with a light at the end through which you pass into the Spirit Plane.
The Spirit plane follows the geography of the real world, but is defined by the perceptions of the things which live in a given area. Areas are defined such that they are proportional to the density of creatures perceiving them so that areas very dense with life (e.g. rain forests) cover very large areas, while barren areas (e.g. deserts, ice caps) are very small, and in some cases parts of them disappear. The relative areas of different parts shift with changes in the distribution of creatures within them. The resolution of an area is defined by the perceptions of the organisms perceiving it. Plants give very blurred, low-resolution images so forests generally show little detail, humans the most high resolution so cities tend to be very well defined. Real world creatures are visible there, but they are conglomerate representations of what other organisms (as well as themselves) perceive them as. This means that an organism can be invisible if nothing can perceive it (though parts of itself it is aware of will probably be visible). Invisible beings and attacks will tend not to visible on the spirit plane. Also, spirit representations of the real world can be somewhat surreal and hard to interpret due to their being filtered through the perceptions of other living things (e.g. words and voices will only be heard as such if they are perceived by something which comprehends them; if not they will be incomprehensible). It is possible for things to escape from the collective unconscious and wander the spirit plane. Some of these may be able to interact with the real world [the origin of poltergeists?].
There is no day-night cycle on the spirit plane (although the 'sun' rises and sets). Objects are visible by their being perceived, but have no shadows. Their colour depends on how they are perceived. The sky is dark apart from a (non-luminous) sun, points of light which can be calculated as Jupiter and Saturn, a few stars, and a large (about twice as big as the Moon) fractal swirl; this is some kind of living super-galactic megastructure millions of light years away. There is no moon. All objects visible in the sky are life bearing. The quality of the 'light' in an area, and its general 'feel' are defined by its emotional ambience in the real world. 'Positive' emotions give brightness, 'neutral' ones silver-greyness, and 'negative' emotions feel dark. Death camps, for example, are truly horrible. No non-intentional (i.e. non-voice) sounds exist on the spirit plane. There are no echoes. There is also no perceptible weather, though the spirit representations of objects may show the effects of weather conditions.
Humanity is starting to dominate both the Spirit and unconscious planes in the same way as it does the real world - humans are becoming more plentiful and their perceptions, because of the intelligence behind them, tend to dominate those of other organisms.
The collective unconscious absorbs the minds of all organisms as they die. However, their minds do pass through the Spirit plane before they are absorbed and it acts as a 'stopping place' for them. The spirits of the organisms wander the Spirit plane for a while before gradually fading as they are absorbed into the worldsoul. This fading occurs more slowly if the organisms death involved strong feelings and emotions. Unforgiving and highly traumatised spirits are particularly persistent and tend to remain in the area of their death. Death camps, battlefields etc. are particularly densely populated by disturbed spirits. Death camps are notoriously bad for this - the huge amount of trauma and the large numbers of people involved give them a different quality to a battlefield, and also make it likely that there will be at least some hauntings.
Earthly ghosts could be proto-superhumans who could have had magical powers, and whose spirits survive death, but without going to the spirit plane or merging with the worldsoul. Hauntings arise partly from these, and partly from psychically sensitive individuals seeing into the spirit plane.
It is possible to retrieve the minds of the dead from the spirit plane if the person has not been dead for long. However, there can be problems with this. This is because processes such as retrieving the minds of the dead, cloning, mind duplication (which is possible by various 'weird science' means), growing lifeforms artificially in bottles, and artificially-created life in general, all cut the being off from the spirit world and the collective unconscious. This makes it harder and harder for them to dream, and unless they are very strong willed they will become "soul-less" and inhuman as their minds drift away from the collective unconsciousness. It is possible to 'reconnect' a mind to the unconsciousness, but this is a very hard job, the details of which are best left to the individual GM...
The Nazi's are very much into mysticism, especially since the appearance of superhumans. It is possible that some of the death camps were created as necromancy experiments. A faction of Nazi mystics could be trying to create an ultimate man using a combination of weird science and magic. Should they come to know too much about how magic works, there could be a Nazi plot to pollute and take over the worldsoul (making everyones unconscious conform to the Nazi view of how the world should be).
Travellers to the Spirit plane from the real world look like their self-image (which may or may not resemble their physical body). They always maintain their own size regardless of the scale distortions of the spirit plane. They are solid and visible to beings on the spirit plane, but are instantly distinguishable from beings 'native' to the spirit plane - they seem to 'glow' with an internal light, whose colour is governed by their internal emotional state (in a similar way to that for the general ambience of an area).
In game terms, on the spirit plane all primary physical statistics are equal to the beings Ego. Secondary statistics are defined partly by the primary statistics, and partly by the persons self-image (e.g. PD and ED will be higher than their base values if a person considers themselves to be tough). They have a 'Dream Pool' VPP of 3 x Ego points in a similar way to that described in the Dreamzone section of 'Champions in 3-D'. Of all superhumans powers, only mental powers are retained on travelling to the Spirit plane, and they automatically gain the 'Visible' limitation. Other powers can be simulated using the VPP, including movement powers and the like.
Giant monsters exist as ancient artefacts reactivated by the same supernova which created the superhumans, or by the proximity of superhumans or proto-superhumans. It is possible that there are magical artefacts hidden in various places around the world, such as Egypt (mummies would be the obvious choice) or south American pyramids/tombs, all waiting for the right person to come close and activate them.
The sea serpent which appeared in the 1940's is an ancient device to defend the seas, which was sleeping as a coral atoll in the Marshall Islands group until it was woken by a survivor from the battle nearby. Hrist the valkyrie is a person who was changed by a reactivated magical artefact.
Magic sight shows that all the stones of ancient sites such as Stonehenge have magic auras, even the missing ones!
Examples of people who could be Mages :