The Buffyverse is pretty much polytheistic given that we've met both one of the Powers that Be (Jasmine, though Angel had interventions from them after her death) and Glorificus. The Powers that Be were probably the closest the shows ever got to depicting a God figure, except it was in the plural.
Jasmine was basically a riff on the antichrist, but it's not disputed that she almost certainly was heavily involved in the visions, manipulations, immaculate conception/impossible birth (Wesley and Lilah even have a discussion about that existing in many myths, not just Christ, as well as the Judas comparison in their Dante's Inferno conversation) and Angel's other miracles.
The First Evil is not Lucifer and almost certainly not an archangel in the Buffyverse, as there's no evidence he or any angels exist in canon outside of one sketchy non-canon comic run.
The mythology laid out by Giles in The Harvest alone outright negates the biblical world building by having the Old Ones (demons) exist before humans and it was hell on Earth, not heaven. Demons explicitly predate humans in the Buffyverse, which is not the case in Genesis. Leviathan and Behemoth are the primordial creatures from before humans in Jewish mythology. Basically, big monsters. The Old Ones either killed themselves off (some with sarcophagi in the Greater Well) or were kicked out of the dimension. All remaining demons are impure half-breeds. The Mayor becoming snake-alicious and Illyria's giant, tentacled Cthulhu form are what Old Ones look like.
In Supernatural, demons are just damned human souls (lots of them victim to making 10-year deals with Crossroad demons) and fallen angels don't become demons either. Angels can have their grace cut out of them (they're rendered mostly human in their vessel) and their wings can end up looking like Castiel's did later (skeletal with molting feathers), but they're still part of the angelic species whether they have their grace or not. Castiel got nerfed hard, losing time travel and flight entirely. Even his healing was malfunctioning.
In the Buffyverse, demons are just a variety of different species, some of which wouldn't hurt a fly like Lorne. Vampires are one of many (parasitic body-snatchers with no unique personalities who must pervert what they get from the dead human host). Vampires are considered dirty half-breeds by purer demons because they're afflicted with humanity and they just think with their stomachs most of the time (hence all the stupid stake fodder). If your I.Q. is as low as Harmony's, the vampire parasitic demon doesn't have much to work with.
In Lucifer, demons are instead the Lilum, or children of Lilith. Lilith was the first wife of Adam made as an equal, rather than subservient Eve made from his rib. She's part of Jewish mythology, but has been excised from Christianity. She's the mother of demons. Supernatural did their own take on white-eyed Lilith (likes to take the forms of creepy children and raises Lucifer from the Pit).
While the show is full of Christian motifs due to it being part of the vampire iconography and Angel's own story (all that Catholic guilt) being a take on the Prodigal Son who becomes the father to another Prodigal Son, the First Evil is just a primordial entity in the Buffyverse whose species isn't stated outside of not being a demon.
While Buffy/Angel, Supernatural and Lucifer are absolutely loaded with biblical imagery and themes, the mythologies are quite often critically different. For one thing, a huge amount of the ideas in Supernatural come from the apocryphal Book of Enoch. Angels and demons alike in that show are body-snatchers who can't use their real forms on the human plane.
Also, the comparison of Lucifer, who, if you consider him the same as Samael, was responsible for giving humans knowledge of good and evil (arguably the very foundation of free will and not being a mindless meatpuppet!), to Prometheus, the Titan who was given eternal punishment by Zeus for giving humans the knowledge of fire, is not a new concept. The comparison is explicitly referenced in John Milton's Paradise Lost. Obviously, Prometheus had much better PR.
Neil Gaiman was responsible for the Lucifer comic the show is loosely based on and the humanist reading of Lucifer being something akin to a Prometheus figure is definitely in there. He's a Prodigal Son raging about free will and ultimately learns that he's self-actualizing all the things he blamed his father for.
The myth of Eros/Cupid and Psyche is part of the source material of Beauty and the Beast. The winged god of lust (we get the word "erotic" from Greek Eros), unpopular for driving men to sin, falls in love with a mortal human girl, whose name literally means "soul", who doesn't know who her mysterious, very inhuman suitor really is. Psyche betrays Cupid to look upon his sleeping form (he flees in anger at her betrayal). Eventually, she goes on a journey during which she receives the secret to Persephone's beauty, except as the Queen of the Underworld married to Hades (another Beauty and the Beast), her eternal beauty is death. Cupid resurrects Psyche with a touch of his golden arrow.
If you've watched the show Lucifer, that should sound familiar.
It's Cupid, not blood-sacrifice-demanding cherubim, that you see on Valentine's Day cards. Bible-accurate angels do not look like angel figurines and funerary statues. Angel figurines instead look like Ishtar/Inanna, winged Isis (halos/nimbi originated as the solar discs behind the heads of Egyptian gods), Eros/Cupid and Nike/Victory (complete with her halo-like laurel wreath), all of which are winged humanoids who are what pop-culture thinks of angels as resembling. Wheels-with-eyes ophanim, six-winged seraphim and four-winged/four-headed cherubim (three of their heads are animal--they're also the creatures on the Ark of the Covenant) in their biblical forms look more like the nightmares of H.P. Lovecraft. Another thing the Bible makes clear is that angels are all male (one could say genderless if it weren't for them fathering nephilim with a description of being "sons of God" who clearly fancied the "daughters of man"), so all those pretty statues of female angels are really just pagan images of Nike, not unlike those fat baby Cupids ("putti" in Italian) on a Christian saint's cards.
Supernatural is pretty much the only angel media I've ever seen that ever refers to cherubim in their real form as having four heads (Zachariah says it). With that in mind, Castiel saying his true form is the size of the Chrysler building is a remarkable shift away from the fluffy, cuddly angels we see in pop-culture. And since he's a seraphim, imagine him with six wings. Note how many human genocides angels commit in the Old Testament!
As for Supernatural, the Lucifer there is 100% all bad, but so are 99% of the entire angel population in that show. Even demons like Crowley and Meg and a witch like Rowena are more reasonable/useful/redemptive than the mostly villainous angels and God alike. Castiel and Gabriel (who spent several seasons impersonating Loki as the Trickster, not to mention hitting on Kali and Rowena) are easily the most pro-human angels in the show's mythology and they're the rebels! And the show ends with the nephilim son of Lucifer (half-human/angel offspring who caused the flood in the Book of Enoch and their Grigori/Watcher fathers became the fallen angels; one being Azazel!) becoming the replacement God.
Castiel is the more nuanced fallen angel, who goes against that 'verse's unjust God a.k.a. Chuck, but was also responsible for killing most of the angels and unleashing a horde of human-eating Leviathans for a year. That was back when he was often gullible, had good intentions that went horribly wrong and was utterly clueless about how to function in human society. He's a take on The Little Mermaid. Castiel got his vessel killed (he got atomized several times), destroyed Jimmy Novak's family and only years later did he realize what an awful thing he did without even considering the human consequences for the man whose body he wears as a meatsuit. He also had romantic feelings for a demon who started off in the show completely murderous, but cared for Castiel when he was insane in an asylum and sacrificed herself to save the Winchesters (not unlike Angel saving Faith and Darla).
Castiel is the one character in the entirety of Supernatural confirmed to have free will. It turns out he really did have a "crack in his chassis" when he was made and he's neurodivergent even by angel standards in the show. The Winchesters were just playthings of Chuck while he plays author, but he didn't control Castiel and that made the difference in the dimension. It's an angel-with-free-will story in a mythology that has no free will by design. His free will was a mistake. Angels are not supposed to have free will and fallen angels are the consequence of them trying.
Note that when Angel has his final words with Jasmine, he makes it very clear that he is on the side of human free will (despite not even being human), regardless of her being correct that more people will die by exercising that free will than she consumes. She made humanity mindless meatpuppets with no free will to accomplish "world peace". Lilah then congratulates Angel for ending world peace. In the case of all three of these characters, their stories take the journey of "The road to Hell is paved with good intentions." Angel, of course, is often called "the demon with the face of an angel" (a description that fits him well) and his very name came from what Angelus did to his family (see The Prodigal). He's also a character defined by the whims of fate playing cruel cosmic jokes on him. He's Prophecy's Bitch who fights back to reclaim free will.
I recommend not getting into a religious fervor watching these shows, even if it's useful background information for where a lot of the imagery and ideas came from.
