Whenever I'm hunting for the best 'Arknights' fanfiction, I usually start at Archive of Our Own. AO3's tagging system is a lifesaver — you can filter by characters (Doctor, Ifrit, Exusiai, Ch'en, etc.), rating, language, and even specific tropes like 'alternate universe' or 'hurt/comfort'. I find the kudos and bookmarks a decent signal for quality, and the series feature helps when an author writes long multi-chapter arcs. If I'm picky about content warnings, AO3 makes it easy to avoid surprises, which is huge when you're reading late at night on a commute and don't want to be blindsided.
Beyond AO3, I often check Pixiv's novel section for Japanese originals and translations, and Bilibili for Chinese translations that sometimes don't make it to English platforms. Tumblr used to be my rabbit hole for one-shots and headcanon threads, and now a lot of authors post links on X (Twitter) or in their Discords. Speaking of Discord, small community servers often have a 'fanworks' or 'fanfic' channel where people drop recs, translations, and updates — I found some hidden gems that way that never hit AO3.
A little habit I recommend: follow authors you like, leave a comment or a tip if they accept it, and use the bookmark/reading list features so you can binge later. If you want recs, search tags like 'Doctor/Operator relationship', 'canon divergence', or 'fluff' — and don't be afraid to try different ships or AU concepts. I love discovering a quietly amazing three-chapter fic that perfectly captures an operator, so give some lesser-known writers a shot; those unexpected reads are the best kind of treasure.