All of these spirit worlds are made out of "ether", and they pervade the material world like gravity does. It is like saying, these spirits are made out of forces of gravity, except their physical influence is much weaker, and their presence within the material is more subtle. A Shadow being might cause a chill when moving thru a material Human. A Fey moving thru a Human would be more exhilarating and wondrous. Both kinds of spirits can feel "spooky" or benevolent. Only a powerful mind, or a desperate one, can exert more physical influence, such as poltergeisting material objects or manifesting temporarily in ghostly apparitions. (Spells handle most of these kinds of mutual planar interactivity.) Most of the time, these spirits interact with each other normally, via virtually physical bodies made out of ether, but can only observe the Humans of the material world going on around them.
All of these spirit worlds are the Ethereal Plane, greater, that overlaps the Material Plane. Except Fey and Shadow are different "frequencies" of ether. Because of this, the Feys at higher frequencies and Shadows at lower frequencies pass thru each other, unaware of each other, even tho both plainly engage the same material world. The "Quintessence" is the Ethereal Plane, proper, the part of the ether whose frequencies are the same as the frequencies of the Material Plane. The Feys are at the energy frequencies above the Quintessence, and the Shadows below the Quintessence.
Elemental spirits typically operate at the Quintessent frequencies, and are part of the natural features of the Material Plane. So the energy vibrations of a particular mountain would be an Elemental spirit that could manifest as a Giant. If poltergeisting the rocks, streams, and plants of the mountain itself, the mountain might be an Earth Elemental. Different features of nature tend to a have different personalities, and different spiritual influences, thus manifest differently when becoming atypically active.
The "Paranormal" researchers investigate reports about unusual spiritual activity. They have electronic equipment to measure the ethereal frequencies, thus can determine whether the spiritual energies that are active are Fey, Quintessent, or Shadow. Likewise they can attempt to negotiate and remedy the reasons for the disruptions. In this sense the researchers function as a kind of technological shamanic.
The electronic equipment of the researchers are casting the See Invisibility spell. Possibly some characters are using the "natural" (Martial) power source to do it. The spells ability to see into the ether also identifies which frequencies the activities are happening at. A slot 2 spell is actually a big deal in a subtle magic setting. The slot 1 "Planar Protection" spell (Protection from Evil or Good) is similarly a useful barrier to defend oneself against any hostiles, whether Fey, Quintessent, or Shadow spirits.
I might create other low tier spells for interactions, such as a Ward Room spell that would seal a room to prevent any ether spirits from entering wherever the spirit planes overlap it. This would also prevent Vampires from entering without permission, and other spirits as well.
To detect immaterial spirits, I would use skill checks. According to the "Study [Action]" entry in the UA Glossary the skills apply as follows:
• Arcana = Aberration, Construct, Elemental, Fey, Monstrosity
• History = Humanoid, Giant
• Nature = Beast, Dragon, Ooze, Plant
• Religion = Celestial, Fiend, Undead
• Investigation = gadgets
This arrangement is probably functional as-is, but thematically I want to finetune it. There is no "Humanoid" in this context. For example, a Human is a kind of sapient Beast, while an Elf is a kind of Fey. Something like:
• Nature = Primal = Material = Beast, Ooze, Plant, where Dragon evolved from a snake
• Arcana = Arcane = Ethereal = Elemental, Giant, Fey, Undead (Shadow)
• Religion = Divine = Astral = Celestial, Fiend, Aberration
• Insight = Psionic = Soul = ki bodily aura, personal ethereal telekinetic influence, mind
• Investigation = Physical Sciences = technology, electronics, math, structural engineering
• History = Human Sciences = cultures, languages, psychology
Note.
These skill checks allow normal Humans to detect and "hear" the spiritual activities of the corresponding Creature Types. A Difficult DC allows a Human to pinpoint and even see the spirit.
Alchemy relates to various skills − Nature, Arcana, Insight, Medicine, and Investigation − and is an ecclectic cross-discipline paranormal technology.
A Humanoid, Construct, or Monstrosity might be any of the above Creature Types depending on origin, such as Beast Human, Fey Gnome, Beast Owlbear, etcetera.
Many Dragon species are nonsapient, such as Drake.
Here Elemental and Giant exist at the "Quintessent frequencies" of the ether. Most animistic nature beings are a kind of Elemental or Giant, but can manifest in the forms of humans or other animals.
Undead stands for any kind of Shadow creature, whether actually formerly alive or not. Since Fey and Shadow are all part of the same ether, it is possible for an individual to migrate, thus immigrating into an other frequency. Probably Seelie and Unseelie are Fey and Shadow (aka Undead) respectively. Perhaps, the Shadar-Kai are by definition the Unseelie. Likewise, Goblin is Shadow rather than Fey, and are part of the "Unseely Courts". Conversely, occasionally the "ghost" part of the soul residue of a former Human becomes Fey rather than Shadow. Generally, forces of life and new possibilities are Fey, and forces of death and entropy are Shadow. The personality of ones own spirit causes what part of the ethereal frequencies one oneself is part of.
Aberrations are the part of the Astral thoughts that the multiverse fails to make use of, thus is more like a cosmic collective unconscious. These thoughts are unhelpful to the operations of the fabric of reality. New thoughts, rarely helpful, can emerge from there, especially when the thoughts of the multiverse are no longer functioning sufficiently, thus inviting paradigm shifts. I have two views: either Aberrations are the NE Fiend and the Yugoloth are a kind of Aberration, or Aberrations are entirely Unaligned albeit dangerous, like dreams becoming real. If they are "Evil", then they are part of the multiverse, thus a kind of Fiend. Or. They are unorganized by the multiverse, therefore are Unaligned and cannot be Evil, per se, just like a dream cannot be Evil.
The Paranormal Researchers are generally aware of these different ethereal frequencies as well as the mindscapes of the Astral. But most Human cultures think these Paranormalists are silly and superstitious verging on insane.