A few spells mention using spirits, specifically fey, fiend or celestial spirits, usually regarding summoning them to take the form of another creature. A few examples are listed below:
Conjure animals (PHB, p. 225):
You summon fey spirits that take the form of beasts and appear in unoccupied spaces you can see within range. ... Each beast is also considered fey, ...
Find familiar (PHB, p. 240):
You gain the service of a familiar, a spirit that takes the form of an animal ... the familiar has the statistics of the chosen form, though it is a celestial, fey or fiend (your choice) instead of a beast.
Find steed (PHB, p. 240):
You summon a spirit that assumes the form of an unusually intelligent, strong, and loyal steed ... the steed has the statistics of the chosen form, though it is a celestial, fey or fiend (your choice) instead of its normal type.
Some of these I am identifying as fiendish or celestial spirits by inference, although conjure animals explicitly says "fey spirits".
My question is, what exactly is a fey spirit? Or a fiend or celestial one? (If this question is considered too broad by asking about all three, I'll happy reduce the scope to just "what is a fey spirit", but I don't see these as being too different to make this too broad, personally).
The only way I can make sense of this is to assume that this is referring to a fey being that has died, so literally the ghost of a fey. But then why can't some of these spells use an elemental's spirit, or even a humanoid's spirit? But even then, wouldn't these spirits be undead type, rather than fey or whatever, if they were the spirits of dead creatures? Shouldn't spirits of dead fey have gone to some kind of afterlife; how come relatively low level magic can summon them? Maybe it isn't literally a dead fey, but then what is a fey spirit?
Note that I don't expect each of the questions in the above stream of non-bold questions to be answered, they are just included to try to give some insight into my train of thought/confusion.
Also note that, although I'm asking "why" this or that, this isn't a designer-reasons question, this is a lore question, since I'm interested in the in-universe justification for things, not designer reasons. I don't care why, in the read world, Jeremy Crawford or Gary Gygax or whoever else from however far back this goes decided that this is the case, only how this makes sense in-universe. What, from a lore perspective, is a fey (or fiend or celestial) spirit compared to an actual fey (or fiend or celestial) creature?
If a setting is necessary to answer this question, let's assume Forgotten Realms, but I don't actually care about a specific setting. Given that this is a lore question, I'm also happy for answers to include information from previous editions, but I'm primarily interested in information from 5e.