D&D also makes a distinction between a soul and an animating spirit, so we could also draw on that. IIRC, the animating spirit is what actually moves the body, and it holds all the memories, but it isn't capable of acting intelligently or learning new information. Most mindless undead have an animating spirit but not a soul. Often they'll mimic the routines they had in life, even when those routines stop making sense (e.g. miming sweeping motions with a broken broom handle with no broom head). They seem to also respond instinctively to the presence of the living by attacking them.
The echo ghost could be another interesting thing to explore, and it would probably have some unique properties of its own. Perhaps they're created by spiritual residue left behind by the dead person, both as a result of their death, but also just by interacting with things while alive. Once they're dead, that spiritual residue might conjure up the memory of that person as a ghost, but the ghost itself isn't really real. The ghost isn't intelligent, per se, but acts according to how that person was remembered by that place or those objects (or some other person that holds a large amount of the dead person's spiritual residue). Over time, such spiritual residue might fade away, or could be cleansed by a priest.
Different undead have different specific powers. Often it's a question of how the undead was created in the first place, as the method of creation can imbue the undead with specific traits. Some creatures are "native" undead, as in they were never alive, and like any other monster they have their own unique traits.How about spawn type undead? how would that work...why
why do some undead drain stats or levels and others don't?
Turn Undead in 5e initially just makes them flee, but as you level up it will automatically destroy weaker undead (it's considered a weak feature, as you get it too late and it only destroys really weak undead). Perhaps Turn Undead creates a pulse of positive energy that disrupts the ability of ghosts to manifest, triggering a primal reaction wherein they feel as though they must flee or be destroyed. The pulse isn't actually that powerful, but it's enough to trigger that primal fear.
monster mash", a collection of Halloween-themed monsters that does include some undead, like vampires and mummies, as well as Frankenstein's monster (which is either undead or a construct), as well as some non-undead monsters. Anyway, I'm not considering werewolves to be undead.
For vampires, there's usually some kind of powerful magic or curse involved, so that can explain why they're unnaturally full of life for an undead. The same magic that causes them to disintegrate in sunlight or running water, or that forbids them from entering a home without being invited, or that causes them to become paralyzed when staked through the heart, etc., also allows them to appear as nearly alive, or turn into a cloud of mist or swarm of bats, or climb up walls, etc.
This could be an interesting thing to explore, why some people die and go to the afterlife while others linger and haunt the living. For undead created by magic, the attachment can be enforced and made permanent, so that no amount of time passing will weaken it, until the undead creature dies again. Generally I would assume that most of those who die go immediately to the afterlife, so those who stick around are probably pretty rare and might not be as well understood as the undead that are created with magic.The sprit grows attachments in life
A typical living person's spirit and body are strongly attached. This weakens after time and especially after a proper burial. Eventually the separation is complete enough that a spirit can move on to the afterlife.
I actually didn't think about this, but it does make sense. A functional brain is probably something that all undead have to manifest, albeit the fresher their corpse is the less work they need to "repair" their brain. A ghost that can't manifest a brain would be extremely limited in what it could do, and might cease to exist altogether. Those that can only manage a partial manifestation might be limited to more instinctual actions typically associated with mindless undead.Thinking requires and articulately manifested brain
The typical undead are doing something akin to sleepwalking. Within seconds of death (and often even before) the brain is badly damaged.
A sapient undead requires extra effort be spent to supplement/replace the damaged brain. A typical lich's phylactery is a complex item that the lich crafts and attaches themself to while still alive.