The Thaumatadiastasis is a plane of reality that connects all the universes of les Plusieurs Mondes and seems to be composed of what can only be described as pure magic. Often referred to as the Dream Realm, it is a place where the laws of physics do not exist and reality itself shapes itself according to the will of the living things that sometimes fall into it.
Magic is what connects, and perhaps creates, les Plusieurs Mondes. It exists outside of space and time.
Sometimes, tears in the fabric of reality of these universes of matter and energy open. When this happens, the corrupting influence of magic enters the world.
It is possible that even in the least magical worlds, there are always a few tiny holes leading to the Thaumatadiastasis and that is why life itself is so common among all the universes.
Scientists and alchemists in dozens of the more enlightened worlds know there is something to this, isolating one simple compound that opens a door into this realm.
DMT  exists in all living things. It is theorized that this compound's ability to break down the barrier between space and the magical realm is what caused magic to enter reality and is why it's present in all life.
This doesn't explain how some worlds experience far more magic than others; DMT levels haven't been found to radically fluctuate between worlds. Some of the erudite hypothesize that a catalyst (most likely gravitational) which opened or weakened the barrier between the world and Thaumatadiastasis interacted with low levels of DMT already present in the physical area. In these cases, left-over weaknesses in the barrier between physical world and Thaumatadiastasis due to the extreme forces at work in galactic formation are cited as possible explanations.
DMT's special connection to the Thaumatadiastasis also begs the question as to the nature of the special connection between magic and biological life itself.
Magic in Practice
For sentient beings In a low magic world, consuming DMT may be used to expand the mind, but In a high-magic world, it can open access to magic itself, allowing the person to wield it in novel ways.
All plants and animals have some small bit of DMT within their physiology, and this is likely the cause of many monstrous creatures in high-magic worlds. Having a surplus of DMT production within the pineal gland is most likely what gives some humanoids a "talent" for tapping into magic, and why a near-death experience often makes them aware of their talent.
In the most advanced cultures within all the universal infinity, none have truly been able to create a reliable way to predict how magic will interact inside a physical universe. In general, however, its presence defies and warps the nature of physics and reality itself.
The magic user him- or herself becomes a conduit between magic and reality and acts as a lens or filter, controlling the how reality is altered by the magic. Psychologists and Neuromancers alike agree on this point.
Because of this, magic wielded by intelligent creatures has no limitation on the changes it can wreak upon reality except for those posed by the will of the wielder. Far more than a few worlds have been destroyed by an unready mind, but more often the unready wielder will die before a leveling more than a few hundred square miles.
To a trained mind, magic can be used as a precise tool. Conjuring a flaming sword, reading someone's mind, flying and making a fancy table set with a 4-course meal appear out of thin air are all possible for the initiated.
The arcane magic spells used by Wizards must be prepared beforehand and then can be cast with a gesture or short incantation. Magic users must practice and hone their ability to store increased numbers of these spells.
In truth, these spells could be said to be stored in pocket universes within the Thaumatadiastasis. The brain of a magic user can only maintain so many open connections at a time. The various incantations used when casting a spell are much like crutches that help the wizard open the connection and unleash the spell. Expert mages tend not to need these crutches.
Divine vs Arcane
Divine magic is typically the result of powerful spells created by very high-magic beings within a universe that reside permanently within a magical pocket dimension. Unlike spells cast by Wizards, these spells don't disappear after being used and can be used by any to whom the deity grants access.
Like the wizard, a priest is limited to the number of connections they're able to hold within their brain and can increase the number with training and natural talent. However, their connections are to eternal divine spells and so they create each connection by communing with the deity. A wizard creates both a temporary pocket dimension and a connection to it when they "memorize" their spells using a spell book.
Magic as a force within the physical world has the power to physical alter reality and as such is often used to imbue particular physical objects with a residue of its power.