Hello!! First off, thank you for your words!! I'll try to answer the question about worldbuilding as best as I can, and give off a few lore drops from my very dusty and full of cobwebs 24 worldbuilding gdoc.

While I have a firm belief for myself that my stories should have three different ways to be read (as in, I should have enough in it and in myself to give three distinct new things per read or reread), that's not necessarily how it starts. 24 started as a small thought from two distinct Jacob Geller:
1) Control, Anatomy and the Legacy of the Haunted House (https://www.youtube.com/watch?v=mexs39y0Imw)
2)Fear of the Depths (https://www.youtube.com/watch?v=7MOKTU9tCbw)
And also, although in a much minor way also this one: "Why do horror games sound so beautiful?" (https://www.youtube.com/watch?v=6aFfN4HtjEc). I've had my current dear editor both refer to Anatomy of a Haunted House and a VERY specific song in the Horror Games Music one as something that reminds them of 24, unprompted, which made my whole year tbh.

Another thing that was important to the creation of the worldbuilding for 24 was Occultism. I promise that is as annoying as it sounds. I won't reveal ALL occultic/esoteric references, themes and usages (and don't worry I'm not using anything demonic), but, for example, if you're curious, look up the Seven Hermetic Principles - I think you'll see some things that could be applied to 24, both in particular scenes and as a whole.

You asked about the worldbuilding, and here I am giving you a rundown of how it started and its main inspos. I do hope you forgive me, I'll go right into the worldbbuilding in a second, but these elements are inseparable of the overall narrative. Why? Because 24, beyond some personal demons I'm exorcizing as I write it, is about the chase of the sublime, alienation (in many forms), and the consequences, or cost if you will, of both of these. (Not that it's perfect, mind you, I do make lots of typos and mistakes and even plot holes/lore holes - I am just one person, and in truth, I hope to one day review the series entirely after it's finished to have it all be consistent. My apologies for mistakes in the meanwhile.) With these in mind, and the references I already mentioned, the worldbuilding comes naturally: a power system that requires balance, is the manifestation of will (but also not), and the chase of perfection. Or, as you know it in 24, Truth.

How do I portray alienation in a magical world? I make the people capable of using magic be stuck in an unknowable place where most can't get out of. I take away their humanity, I crack them open or in two, I call them Residents instead of people, I make most come in young so they forget, I make the ones that remember the outside and their communities die or suffer. I call people objects (Gates, Books, Bridges). I don't give them relationships that fit in traditional molds. When they have human like reactions, I call them animals. Etc, etc. I'm sure you can think of more examples - I will admit you have read 24 much more recently than I, hehe. I do try that these things are made to be as "natural" within the world - I am a try hard writer that attempts to make the story both as much from my pen as from the world I build, I don't separate worldbuilding from my writing, per se, but both happen simultaneously.

How do I portray the chase of the sublime? Well, for a very concrete example, I have Taeyong be consumed near-entirely by this otherworldly thing called Magic, or The Place. In the scene where you see him, star made, he mixes with the Place itself, he's almost entirely his element. This chase also happens with Donghyuck, this consumption of books, his limitations as he's bound to the material plane.

How do I portray the consequences? Well.

As for tidbits!

Mark is able to verbally speak, he just chooses not to, or is overwhelmed by speaking in most circunstances. I was doing research in Selective Mutism for something entirely unrelated to the story, I think, and I came across it and thought that while it didn't fit the specific thing I was looking for, it did indeed fit this damn character pretty well!!! However, since I didn't find many resources in Adult Selective Mutism, I did add a bit of personal experience here and there.

My favourite chapter to write was chapter one, as I could go buckwild in the description of The Place. The Place is HUGE. There's three more Towers in the Place, but the Place is so big that communication between them takes ages, and there aren't a lot of interactions from Tower to Tower, they're mostly isolated and each do their own thing. (Feel free to imagine your kpop boys living in other Towers! Hehe).

Renjun's culture and Jaemin's culture historically just straight up don't get along. I don't think they have, however, realized that the Place distorts time immensely just from existing, so there is a possibility that a) their cultures have changed and a new reality emerged or b) their cultures just straight up don't exist anymore. Bridges are the only ones allowed to leave The Place, and Gates, in very very very rare occasions, but since The Place is so atemporal, and Specific outside info relatively restriced, these time shifts and breaks aren't as easily noted.

The Learning Place is in the center of all maps, mayhaps even the world, and every populated region (The Seas, The Cities, The Far-Aways) is named after the cardinal position determined from the Place. So for example, the four major cities are the City of the West, the City of the East, the City of the South and the City of the North, the same for the City of the Southeast, the Far-Away of Northwest, etc. The Wild Lands are don't follow this naming pattern because they reject the authority of The Place, while the Calmed Lands are named that because they have been tamed. The closer you are to The Place, the less you see aglomerates of population - The Calmed Lands are literally devoid of humans, and cannot sustain normal life.

I've always thought Donghyuck/Haechan to be from The Seas, but I haven't decided each one. I really like associating Hyuck with watery things. Gods know why.

Anyways, apologies for this huge essay. And apologies for the pretentiousness - I'm sure me busting out the Western Occultism bit was a wee too much. I do hope you were at least entertained, and I hope I succeded in answer your questions, even if partially.

And, btw. New chapter before the start of March.