I don't know where to look, either. Any suggestions? That generation really needs an alternative to time wasting video feeds. My girls (16 and 12) are either on TikTok or Webtoons. Thankfully more Webtoons, but even that's not ideal.
Truthfully, I'd say there's probably not a single source for meaningful content for kids that age. Different interests have different community hubs. For example, the skincare community used to have a lot of interesting scientific discussions on /r/skincare or /r/AsianBeauty, but honestly it's become less intellectually engaging in recent years (the scene was probably just poisoned by too much commercial interest). There also used to be interesting discussions on /r/femalefashionadvice, but that's also died. Interesting discussion of fashion has migrated mostly to YouTube, but even there it can be hard to find meaningful content that isn't just a string of sponsorships.
From what little you have written here, I'd guess that by their interest in Webtoons, they'd be interested in art, storytelling, and maybe some Korean culture stuff. The art community on Instagram can actually be quite fun and enriching; many of the artists post tutorials on how to recreate some of their pieces. Perhaps you might encourage their involvement in that? There is a Korean company called Class101 that offers a lot of art classes in styles that would appeal to someone who's a fan of Webtoons (many of the classes offered by Western companies such as Skillshare are in a different style that might not appeal). It's a little expensive (they ship you the materials needed as well), so maybe you could let them pick one they want for a birthday gift or something. I personally tried it and had a great experience.
For storytelling, honestly, Tumblr is still pretty good for that. There are blogs where people will post excerpts from various authors and novels, and you can just scroll through and see if any resonate. There's also some overlap there with "studyblr"/"studyspo"/"studygram", which is basically an online subculture of teenagers and college students who are passionate about making their school notes aesthetically pleasing. Many of the girls at school had a soft spot for this, and it was a fairly wholesome and encouraging community.
For Korean culture content, are they interested in Kpop or Korean dramas? One of the most popular dramas in the past few years was called Start-Up, which (surprise surprise) was about a tech startup, heh. The show depicted various tech projects and demos, and someone wrote some tutorials for recreating them, which your daughters might enjoy trying out if they liked the show! [1] If it's Kpop they're into (which also has overlap with Tiktok), maybe they'd enjoy trying to learn the dances? A lot of dance studios offer various Kpop dance workshops these days, which they might enjoy. Also, they might be interested in learning Korean, or any other language, in which case there's thriving communities for that, as well (this also has overlap with "studyspo" as described above).
Hope this helps! Let me know if they have any other interests; I might know something about it.
[1] https://guolikewhoa.medium.com/creating-all-the-tech-project...