D&D 5E (2024) Aasimar is a really strong species in 2024

[Lazybones]

I don't have a specific aasimar culture in my campaign world, and celestials don't really appear on the material plane that often (barring exceptions like certain familiars and spell summons). So aasimar are "divine touched" or some such, and are only occasionally born to the various other species. I let the player determine how distinctive their heritage features are (i.e., whether they can "pass" as their birth species).

 

[Corinnguard]

3e's Manual of the Planes does provide suggestions on how to have a cosmology that lacks the Inner and/or the Outer Planes. If your setting lacks an Inner Planes, then the Genasi could be individuals who were exposed to areas of the Material Plane with strong elemental tendencies. If it lacks an Outer Planes, then it's areas with strong Celestial or Fiendish tendencies for the Aasimar or the Tieflings. Hallow or unhallowed ground, underground in a place like Eberron's Khyber, or the realm of a deity (like Mount Olympus in Greek mythology).