The obvious choices are Archive of Our Own and Fanfiction.net, but I've found the quality varies so much it gives me a headache sometimes. AO3's tagging is a lifesaver for filtering, you can really drill down to 'Jinx & Reader' or specific dynamics, but you have to wade through a mountain of Vi/Caitlyn stuff first. Some of the most unsettling, character-accurate Jinx voices I've found were on Tumblr, buried in reblog chains from like 2021. Writers there get her manic, fragmented internal monologue in a way longer fics sometimes smooth over.
Honestly, a lot of the best stuff lives in Google Docs links shared on Discord servers. The barrier to posting publicly feels higher there, so what gets shared is often more polished or wildly experimental. I followed a 'chronically online' styled fic told through Hextech chat logs and corrupted image files that was phenomenal, but you'd never just stumble on it.