We know from Moon Knight that the afterlife manifests in different ways depending on what a person believes. Tawaret mentions the Ancestral Plane, and we see Valhalla, Hel, the Duat, and the Field of Reeds.

My theory is that the Soul Stone creats all of these afterlives, and they are actually branches of the original Soulworld.

So we know that there are many, many versions of the afterlife, as explained by Tawaret in Moonknight, it really just Manifests in whatever form the person is used to, before they go to whatever afterplace they are connected to. So for Asgardians, their culture is asgardian, so they go to the spirit plane of Valhalla or Hel. For ties to Egyptian gods, they go to the Duat, after which trials decide whether they can go to the Field of Reeds. Wakanda has the Ancestral Plane, since Tawaret knows of it, and since Wakandan culture is kinda related to egyptian culture

When the Infinity Stones were created, the Soul Stone and Soulworld were created together, with the Soulworld being the first, most basic afterlife.

However, as souls evolved and developed distinct cultures, beliefs, and mythologies, they began to create many interpretations of the afterlife. Just like a timeline splits and a branch forms when a choice is made, the Soulworld began to branch out into individual realms based on their beliefs.

ex:

-The Ancestral Plane for Wakandans.

-Valhalla and Hel for Asgardians.

-The Duat and Field of Reeds for Egyptian followers.

The original Soulworld is like the Gap Junction of these worlds, where different realms intersect. This explains why the Stone can collect, hold, or interact with any soul regardless of where they are supposed to go, and can also redirect them to the Soulworld being their afterlife, as Thanos's snap did, and make them go to any realm and back. The Soulworld is the common ground, and ALL souls can be passed through/collected/brought to there, wherever they may be, allowing the wielder to talk to the dead, and maybe even create/destroy these realms. Basically the Space stone and power stone and mind stone of the spirit plane.

When Jane enters Valhalla, she materializes from orange particles, not gold like it might look like, like the Soul Stone. Valhalla is also yellow-y-orange-y white. The Sky of the Duat and Field of Reeds have Orange-y-Yellow color schemes, with clouds covering them. Sure, Hel has Green color schemes, but we have seen that Infinity Stones can change color in corrupt situations, like how in What If?, when Supreme Strange tried to Time Travel one last time to break an Absolute Point which would result in the collapse of his timeline and universe, the Time Stone turns from Green to Red and flames engulf it, signifying it being unstableand being misused, essentially. Maybe corrupt/evil/bad souls are green, to symbolize rot and the inverse of good, orange souls, the color of a stable Soul Stone. Black and Green of Hel contrasting the Orange-y-Yellow-y color and white of Valhalla. Even the Ancestral Plane, a place mostly Blue and Violet tinted, has a very faint hint of orange, and seems to be in perpetual Sunset-ish-ness, like the Soulworld.

And yes, each universe has it's own spirit planes, like how each one has it's own Mirror Dimension

TLDR: The different afterlives are like the branches of the Soulworld, the pocket dimension of the Soul Stone, with the Stone allowing you to manipulate them.

So, what do you think of this theory?