I'm writing an urban fantasy series in which like many urban fantasy series there is a magic-and-powers system hidden from the broader world that the supernatural phenomena of the series run on. However, I'm having trouble providing exposition as to how this world works. Like many of these series, there is a relatable viewpoint character who is new to the supernatural world and at least part serves as a vehicle for readers to learn about the supernatural as the character does. However, I am running into a big problem in that it's hard to figure out how to give exposition given that the other characters don't have an encyclopedic knowledge of the supernatural world and its magic system themselves. They know the bare bones, like basic terminology, a very broad understanding of what they are (i.e., they don't know how the magic system works, they just know they're weird and have powers), and "don't go near those guys because they'll mess you up". But they can't provide a good explanation of where their powers come from or how they work beyond "they just do".
In most series the protagonist after being inducted into the world of the supernatural would get some big expositional speech on how they're part of an aristocratic lineage of vampires dating back to Cain or they're half-angel demon hunters or the ever famous "Yer a wizard Harry", but one of the big novelties of this series is that nobody has any idea how the magic system works. The supernaturals don't know where they come from, and they don't have some venerable pedigree going back centuries or millennia. A good way to put it is it's like germ theory. In 1800 we had no idea that bacteria and viruses caused disease, but that didn't stop humans from getting sick and dying anyway. Like any natural phenomenon it just happens, and people are forced to react and figure it out on their own. Much like how real science doesn't have all the answers to every natural phenomenon, neither does mortal supernatural knowledge have the complete picture (there is a hard magic system in place, but no character has figured out its mechanisms).
Adding to that the supernatural gimmick in my series is weird, almost to the point of silliness. Like, Jojo’s Bizarre Adventure-level weird. It's so weird that one of the things that's mentioned to keep the existence of the supernatural under wraps is that it's so weird that it's almost impossible for someone to try and spill the secret without being looked at like they're completely insane. The seeming silliness is part of the point, the narrative intent behind this is to lower the reader's guard by making them laugh at how silly this is, only for them to stop and be forced to reconsider when the consequences of this “silly” magic system result in mass loss of human life.
However, this in turn causes two major problems. One is that it's hard to introduce the magic system in a way that the audience can connect to, because there's little pre-existing mythical tradition like there is for vampires, werewolves, wizards, and other supernatural beings that have saturated pop culture. By contrast, in works Harry Potter all Hagrid has to do is say “yer a wizard Harry” and the audience immediately knows what he means, even if they’ve never read Harry Potter book before. It is implied that many myths and legends throughout human history are misinterpreted retellings of this supernatural gimmick (as is typical for many of these types of stories), but none of them are so straightforward that you can say "these are vampires, but different".
The other downside is it makes it really, really hard for the characters to provide exposition in a believable way. For example, I've tried writing the "intro to the supernatural" speech for the protagonist several times and each time I keep finding myself struggling to come up with a believable reason of "why does the protagonist even believe a word they're saying, and why doesn't he think they're just insane?" That's not even getting into when the characters try to explain the situation to other characters. The only thing I can think of that would make someone believe would be is if they got mauled by a supernatural monster first.
Even if the protagonists give a downright inaccurate, one issue is that the audience depends on the characters to provide context to the crazy fictional world they're viewing and so have a tendency to believe what is said without reservation because they have nothing else to judge the characters' words against. This is one of the reasons [REDACTED] in Knights of the Old Republic II, the idea that most of the exposition you received in the story was wrong was shocking. However, this also has a tendency to upset audiences, especially when it comes to the magic system, because it turns out that all the rules of the plot the audience was following are either not quite correct or outright wrong. Rather than getting the impression of "this is what the rules were all along", the audience tends to throw a riot because it comes off as the author changing the rules mid-story.
Given this, how can I provide exposition for the audience when no character has an extensive, objective knowledge of how the magic system works?