Archive of Our Own is basically the central hub at this point. It's where most of the serious writers post their multi-chapter stuff, and the tagging system means you can find exactly what you're looking for, whether it's 'Frozen' Elsanna angst or some obscure 'Gargoyles' deep cut. The quality tends to be higher there, in my opinion, because the culture is more focused on writing as craft.
That said, you can't ignore FanFiction.Net. Its interface feels like a time capsule from 2005, but the sheer volume is staggering, especially for older fandoms like 'The Lion King' or 'Hercules.' I still check it for stories I remember from a decade ago that never got cross-posted. For newer stuff, though, it's a bit of a ghost town compared to AO3.
Tumblr and Wattpad are a different beast. Tumblr is great for headcanons, drabbles, and super niche ship dynamics that blossom in the tags—I found this incredible 'Encanto' Bruno/Mirabel artist there that would never fly on the bigger archives. Wattpad skews younger and loves those 'Y/N' reader-insert stories, so if you're into self-insert romances with Prince Hans or a reformed Gaston, that's your spot. The algorithm can be hit-or-miss, though.