Souls, Ghosts, and Afterlife
The soul is the entity created when primate (or greater) level reasoning ability is combined with variant independent motivation (aka emotions) within a single structure. It’s impossible have such a structure and not have a soul. Everyone has one. Humans, vampires, werewolves, the works. It’s naturally created when a child’s brain matures to the point of mating the reasoning process with emotions, but in rare cases like Prometheans (or theoretically sentient computers), it’s created with the being.
If a child is at the point where it’s about to form a soul, a disembodied soul may easily possess the body, obviating the need for soul creation. This is how reincarnation works. When the soul takes the new body, it becomes a fundamentally different person. The mind of the new host doesn’t possess the memories, abilities, personality or anything else possessed by the soul. It has the opposite effect. The mind imprints on the soul. A soul that’s been through multiple imprintings can draw on the knowledge and experience of any of it’s lifetimes, but only when not in a body. The primary personality at that point can switch, and generally the lifetime that led to a person with the strongest sense of self is the main personality for the ghost.
A soul without a body is either a ghost or a ‘wisp’. Ghosts work as normal. A wisp is how NWoD canon describes souls: faint ghostly presences in twilight, but without any strength or corpus as a ghost would have. Mages are generally accustomed to a few of these flicking by in the background while looking for something else with mage-sight. The main difference in this universe is there is a canon explanation as to where these wisps go when they fade out. Once they lose their grip on the living world, they descend into the Underworld and gain the characteristics of a weak ghost that then has to deal with the Underworld on those terms.
The Underworld works as described in the Book of the Dead, as described here. The main difference is that the size is based on the number of residents; ever expanding.
A body with it’s soul stripped from it generally doesn’t have the ability to generate another soul before undergoing metaphysical collapse as described in the Mage core book: losing dots in the morality trait, then willpower, then a catatonic madness.
Vampires and Prometheans are normally described as being without souls. In this universe they have a different problem; their souls are inextricably bound into their bodies by the process of the Embrace or Creation. Vampire elders have a range of tricks they try, Cruac sorcery, Draconic ritual, Sanctum redemption quests, etc. Sometimes they even work. Prometheans are generally on an extended pilgrimage to regain their humanity and thus the body/soul divide.
Helpful geists aware of the true nature of reality tend to be less about dismissing ghosts and more about shepherding souls in twilight to suitable hosts, protecting soul-ghosts (not the broken echoes) from having their anchors destroyed, and similar acts of spiritual do-goodery. Strong geist krewes frequently travel into the underworld to try to make life better for the billions on the other side.
Supernaturals tend to try to hedge their bets against their mortal end. Either preparing to become strong ghosts, ensuring that their soul always has a new host, or taking the long-shot at immortality. All have their problems.
There are limits to what you can do to prepare for becoming a ghost and there’s no guarantee you’ll turn ghost rather than wisp.
Reincarnation wipes your personality until you die again, and unless you’re part of an organization, you won’t be raised with the knowledge to prepare yourself a second time (a master level mage of appropriate arcana might be able to remove the soul, force it to an appropriate personality, and reinsert it for another person, but could not do it for him/herself).
Immortality, apart from vampirism (which risks oblivion), is hard to come by. Usually it requires horrible sacrifices or constant risks which can often defeat the purpose. No one gets a free ride.