Racial Origins

[I ran out of allowed drafts for World Anvil and this is definitely a Work-in-progress]
One origin cosmology could be that the material plane was created late in the scheme of things. When it was created, beings from the various other planes visited and dwealt there. Offspring conceived and born on the material plane was unlike creatures conceived and born on their home plane, essentially being a whole new race. Fey creatures (the Noble Eladrin) gave birth to elves, dragon eggs hatched dragonborn etc. Each of these new races had a humanoid form, chose their alignment (the progenitor races took theirs from birth and could not change), and would die of old age. In this world, monster races encountered are all extra-planar and have inherent alignments. Orcs and Goblins, for example, will inevitably have a portal to their home plane in their lair. Though they use the lair as a base of operation, no reproduction happens on this plane. They are spawned on their home plane.
Why do they dwell in the material plane, and why do they raid those who dwell there? Perhaps their home plane (the shadowfell?) is more dangerous to them (post-spawning) than even a lair on the material plane. They raid for supplies and also maybe take some supplies back to their home plane to make for more spawning and to earn more underling spawn.
Humans would be the offspring of Devas. There are three types of Devas and maybe the different races come from different combinations. The Devas withdrew from their children when they discovered that the children could choose to be evil, deciding that non-interference was best.
When the Noble Eladrin discovered their material children were mortal, they returned with some of them to the Feywild. Those elves that returned to the Feywild continued to be mortal, but formed the subrace of Eladrin.
Dwarves could come from the elephant-like hollyphant? Or Mount Clangeddin on the first Layer of Arcadia, Abellio. Or earth elementals/Xorn reproducing on the material plane. Or the Galeb Dhur. https://1d4chan.org/wiki/Clangeddin_Silverbeard

Teiflings would be the result of Succubus/Inccubus relationships on the material plane. Their progeniters were shocked to find their offspring could choose to do good. Probably also shocked to have offspring at all.
Dragonborn would hatch from dragon eggs laid on the material plane. Their colours would probably match that of their parents. But maybe not.
Gnomes are one of the core playable races, so I would probably give them a distinct origin, but someone just pointed out that they have a possible backstory as half-elf, half-dwarf. Weird. Later they got an origins story as denizens of the Feywild, alongside the Eladrin, but had spent generations as slaves under the cruel dictatorship of the Fomorians; cursed fey giants with powerful magical abilities and mystic eyes. These gnomes worked hard, developing their skills with illusion magic and stealth, and ultimately managed to escape - for the most part. That’s 4e, 5e reverted apparently. Maybe they are the offspring of earth elementals vs dwarves as offspring of Xorn. The Treatise Historical of the Dragon Tyrants held that gnomes were created in approximately −24,500 DR from gems hidden in caverns beneath a mountain in the land later known as Netheril. https://www.realmshelps.net/charbuild/races/gnome/gnomes.shtml  “Before there was time, before there was anything, there was nothing, and before there was nothing there were monsters” - Sweetpea
So, creation starts with the primordials. The primordials explode outward into a ring of planes. They populate the planes. One primordial is the proto-dragon. No colours, or all colours, it travels from plane to plane, laying eggs which hatch into dragons of a colour/metal based on the plane (good planes get metallic, evil planes get chromatic). Other primordials are also about seeding lots of planes, or focusing on one. Outer planar entities of all sorts are created. The planes affect their residents, turning them from formless entities into archetypical beings. Some primordials shun the planes, remaining in the ethereal void. After a long time, various creatures exist and then suddenly a new thing. In the very middle of the planes, between the Feywild and the Shadowfell, in what was formerly an ethereal expanse, appears the material plane. These retain their alien nature. It is disconnected from the other planes and for a long time only the wild and wordless creatures (“plants and animals”) live on it. But with time the two closest planes (Feywild and Shadowfell) begin to grow closer in substance to it. The first to find their way across are the Fey creatures known now as “Noble Eladrin”. They arrive on an island a little distant from the campaign setting (now a sort of grey-haven) and they take great delight at the beauty they find. Moonlight and stars, the brightness of noonday sun, the colours of rainbows are foreign and strange. They settle and reproduce but their children, born on the material plane, are lesser than they - less magical, less graceful, less powerful, but still having their essence. Over the next thousand or so years, the Noble Eladrin discover also that their children of the material plane are mortal - they age, and in time they die. Many of the Noble Eladrin leave and take their children back to the Feywild. Those of their children that stay become the elves, the ones who leave are elves as well though they are closer to their parents and are now known as Eladrin. The elves build ships and sail throughout the land. Eventually entities from other planes find their way to the material plane too. Dragons lay eggs, from which hatch the dragonborn. Xorn lay eggs from which hatch Dwarves (who bear their young, not lay eggs). The creatures born on the material plane demonstrate a much greater degree of versatility than their parents, choosing between good and evil and either accepting or destroying others. Some dwarves and elves take up together and the race of gnomes begins. Powerful fore-runners to the succubi and incubi arrive and produce teiflings. The various races interact and sometimes fight. Their powerful predecessors overwhelm the fights and open portals through which come more powerful entities still. Eventually someone or other (probably Fey) activates a powerful magic and the planes are separated more powerfully. The separation is something like a filter - the more powerful the being, the harder it is to cross. Elves who know the right place can get by, but an archfey would really struggle. It takes a great amount of power for a dragon to cross. Some powerful creatures do remain in the material plane though. The gods themselves have never had access to personally access the material plane, so perhaps the filter was always there, it was just tightened down.
There’s a lot of interest to me here. I’d make humans the newcomers and they have to pick how they will integrate the older races stuff. So there’s a five pointed star, and each point has a god and a major race that god created (elves (green??), dwarves (blue??) … three more?) plus there’s a few auxiliary races that were created by the over god(s) (halflings...) or maybe the auxiliary races were created by interactions between the five-gods. But then humans get created by the demiurge and sit smack-bang in the middle. Dragonborn (white), Teifling (half-black), Orcs (black), gnomes (red??) It might work if it’s a different five-point star than the MtG one, but then I’d have to do my own work on all the interactions.
https://rpg.stackexchange.com/questions/77247/what-are-the-playable-dd-races-in-5e

Summary