Finding stories focused on Jiang Cheng and Nie Huaisang means hanging out in spaces where the 'SangCheng' ship lives. It's not one of the monstrously huge pairings, so you need to know where the dedicated fans congregate. Archive of Our Own is, unsurprisingly, the absolute heart of it. The tagging system means you can filter for their relationship tag and then sort by kudos or comments to find the beloved ones. There's a real depth of characterization there; some authors really dig into the potential of two sect leaders dealing with their respective traumas and legacies.
Beyond that, I'd point you toward specific corners of Twitter (or X, whatever) and Tumblr. A lot of creators use those platforms to promote their shorter works or link to their longer fics on AO3. Searching the ship name or the hashtag #sangcheng often turns up fanart, ficlets, and links to longer stories hosted elsewhere. It feels more like a community gathering spot than a formal archive, but that's where you discover the writers who are really passionate about this dynamic.