Looking for Kim Yn stuff? You've got to hit Archive of Our Own first. No contest. The tagging system alone makes it worth it—you can filter for exactly what you want, whether that's romantic pairings, specific characters he interacts with, or just fluff versus angst. The community there is massive and really active, so new content pops up constantly. I've found some incredibly well-written longfics there that explored dynamics the show barely touched on.
Wattpad's a different beast, but it's huge for a reason. The algorithm seems to favor certain tropes, so if you're into the more popular, dramatic, or trope-y stories (enemies to lovers with a certain co-star, anyone?), you'll drown in options. The comment culture is very immediate, which is fun if you like interacting as you read. My main gripe is the search can be a bit messy compared to AO3's precision.
For something a bit more old-school, Asianfanfics still has a dedicated corner for this fandom, though it's not as bustling as it was five years ago. You can sometimes find gems there that never got cross-posted. Honestly, I tend to check AO3 daily, Wattpad when I'm in a specific mood, and Asianfanfics maybe once a month for deep cuts. Tumblr and Twitter are also essential for finding links and recommendations, but they're not primary hosting sites.