The multiverse of Magic: the Gathering is rather unusual in that it technically has a name: Dominia. Granted, that's some old and probably defunct lore from the mid-90s, but I think it's good to take into account.
Given that two Magic settings are now officially also D&D settings, it might be a good idea to look into the way that these worlds bear resemblance to, but also are distinct from, classical D&D cosmology.
Both Ravnica and Theros might strike you as particularly strange and rather high-concept worlds, and that is true. The reality of building a card game primarily around mechanics, with worlds that are often made to fit with the mechanics, rather than the other way around, tends to make for some very high-concept worlds - worlds that sometimes have you wondering how one actually lives in them. Nevertheless, Magic does have ongoing plots and stories set within it, and while the cards represent the most over-the-top, crazy things one can find in these worlds, the underlying premise is that these are still worlds that people live in, with all the potential for a fantasy RPG like D&D to fit within it.
One thing to bear in mind in the worlds of Magic is that each world is its own separate plane. Admittedly, there's an argument to be made that "plane" is just a different concept in Magic than it is in D&D, but I actually think it works pretty well if you think of each world as a truly separate universe with its own distinct rules - more akin to the differences between Limbo and Arboria than those between Oerth and Toril.
In the Magic multiverse, the ability to actually travel from one plane to another is extremely rare. People who gain this ability are called Planeswalkers. About one in a million people is born with a Planeswalker spark (though if you're running a D&D campaign in these worlds, you might want to ignore these statistics to allow the party to hop between settings) and only one in a million of those ever have that spark ignite to turn them into a full-fledged planeswalker.
Typically, what ignites the spark is an event of profound emotions - often traumatic (though they need not be in every case.) A newly ignited planeswalker will, involuntarily and unexpectedly, travel to a different plane at random.
In an earlier time, planeswalkers were godlike in their power - their physical forms little more than a manifested avatar behind tremendous magical force. But after an event called The Mending, which occurred on the world of Dominaria, Planeswalkers are now more or less just people, though they tend to possess some great magical power and the unique ability to journey between planes.
There have been some exceptions to this rule, but for the most part, only planeswalkers can travel to other planes. Even gods are generally limited to their home plane, which is why different worlds will have different pantheons - at least those whose religions weren't just deifying old planeswalkers.
Beings that would be considered "outsiders" in D&D are probably native to the same planes that have ordinary mortals and beasts. And this is also why angels in one setting might be very different from those in another. Races found on multiple worlds can be quite different from one another. For instance, Vedalken, the blue-skinned, hairless intellectuals, have only internal ears and wider facial features in Ravnica, while those found on Kaladesh have earlobes and vertical lines along their foreheads.
The one constant in the multiverse, though, is the manifestation of the five colors of Magic.
Magic flows through everything in the worlds of MTG, and the magical essence known as mana comes in five varieties, known as the five colors. This is, to be fair, a major element to the rules mechanics of the card game, but it's also an underlying organizing principle for how the worlds of Magic are constructed. The five colors are White, Blue, Black, Red, and Green. The colors represent and also drive forth certain values and philosophies, as well as certain types of magical creatures.
White mana is a color of unity, altruism, order, authority, and selflessness. Found in wide-open plains and associated with sunlight, it's easy to conflate white mana with goodness, but white can often be good, it's also a color of tyrannical authoritarianism and strict hierarchies that have no real interest in freedom. Angels, who in MTG are nearly always female-presenting, are beings of pure white mana who represent justice and protection, tending toward the better aspects of white mana (usually.) Knights and soldiers, often human, are also usually associated with white mana (though humans, being so adaptable, are found associated with all the colors.)
Blue mana is a color of intellectualism, knowledge, trickery, cleverness, and illusion. Found on islands and associated with the elements of water and air, blue mana is the primary color associated with powerful wizards. Blue is of course laudable for its rationality and cleverness, though it can sometimes veer into cold callousness. While beings like Sphinxes and Djinn represent the cleverness blue is known for, it's also a color that can call upon great ancient monsters like krakens and leviathans, using its knowledge of hidden secrets to call forth forgotten beings form the depths. Merfolk and the aforementioned Vedalken are also usually associated with blue mana.
Black mana is a color of ambition, self-interest, greed, ruthlessness, death, decay, and resourcefulness. Found in fetid swamps, black is often associated with that which is buried or sunken, including anything from dead bodies to precious metals (and thus wealth.) Much as white is often assumed to be good, black often gets "type-cast" as evil, and while there's often an overlap, black is not bound only to be villainous. At the heart of black mana is a focus on the accomplishment of a goal, no matter the cost, and as such, when directed toward noble goals, black can be effective and heroic. Still, its disregard for any kind of moral framework means that its methods are often quite distasteful. Much as white mana can manifest as angels, demons are manifestations of pure black mana. Black mana is also used to raise the dead as zombies, skeletons, and vampires.
Red mana is a color of passion, impulse, violence, aggression, creativity, and destruction. Found in rocky mountains and volcanos, red mana drives emotions and rage. More than any other color, red fights for freedom, and while black might fight against structures of authority for its own self-interest, red does so on principle. Red is associated with the element of fire, but also with earth and lightning, and even, very rarely, ice. Red's recklessness can make it a danger to its foes, but also to its allies. The creatures most associated with red mana are dragons, as well as goblins and minotaurs.
Green mana is the color of life, nature, instinct, and symbiosis. Found in the living forests of Magic's worlds, green is a color one might tend to associate with goodness, though like white, this tendency is not a universal rule. Green can grow dangerous when its obsession with raw strength leads the strong to menace the weak. Green is associated with great and powerful beasts and monsters including Hydras, but also with humanoids like elves.
Generally, the colors are depicted in a pentagon-shape, with white at the top, and then blue, black, red, and green making the other corners, clockwise. As you might sense reading the descriptions of these, each color has two "allies" next to it, which have more of a philosophical overlap (for instance, Red and Green are both very action-oriented worldviews) and two "enemy" colors across from it (by contrast, Red isn't such a big fan of White's tendency to try to boss people around.)
And while it's true that there can be some conflict between enemy colors, there are also individuals who embody aspects of two seemingly opposing colors.
The guilds in Ravnica each represent a different pair of colors, meaning half of them represent "enemy" color pairs. But you might also be able to see how sometimes these opposites can complement one another. The Boros Legion takes the order and duty of white and combines it with the zeal and passion of red.
While the colors are the closest equivalent to D&D's Lawful/Chaotic and Good/Evil alignment system, they're less hard and fast in terms of how alignment with one precludes the other.
Within the Magic worlds, most people aren't really aware of these five colors - they might recognize tendencies in certain cultures and creatures, but it's a subtle thing that probably only very advanced spellcasters would truly be able to see the underlying structure.
The planes of Magic are not really organized in any real hierarchy or arrangement like the Great Wheel. While Dominaria is theoretically the "central" plane of the Magic multiverse, its position does not seem to make it manifestly more important than other worlds.
The list of Magic planes is extensive, and also constantly growing, so I'm not going to do an exhaustive list here. But the thing to keep in mind is that each world is governed by the balance of these five colors and what they represent.
In recent times, a group of planeswalkers has formed something called the Gatewatch, which is dedicated to fighting against interplanar threats. Given that this group is essentially an adventuring party, you might want to keep them separated from your own D&D group, though you can also potentially bring them in as powerful NPCs.
