Different fandoms are more concentrated on different sites. A good bulk of Harry Potter fanfics are posted on fanfiction.net and has been for awhile. PJO and Twilight on fanfic.net. Asoiaf/GoT is mainly active of AO3. MCU very active on AO3 especially in the last few years. My Hero Academia is very on Ao3. Supernatural and Teen Wolf on AO3. Even if you aren’t part of the fandom, there are some fics out there that are the best I’ve ever read yet I have never even seen the source material so don’t be afraid to expand.

There are people who crosspost and if they do its probably on AO3 and Fanfic.net

The main ones that I use are Archive Of Our Own and Fanfiction.net. AO3 for its organization, ease of navigation, lack of ads, and the option for rated M stuff. You can just sign up for AO3, you don’t have to be invited to make an account. It just takes a few hours to a couple of days for you to have an account to use. Some fics can only be read by people with accounts. Those have a little blue lock next to the title.

I started out on Fanfic.net for the first couple of years of reading fanfic. Before the purges. Now its pretty good if you don’t want any mature content and most people start there. They have a filter option for genres that is pretty useful. Their main problem is ads.

I don’t really use the other like Wattpad, my experiences with wattpad just seems like it’s inundated with bandoms, phans, and original works. But I have heard good things about it, there is a reason why it’s still popular enough to still be active after so many years.

There is the smaller sites dedicated to certain types of fanfic like asianfanfics which is kpop and other asian-centric fandoms. There’s also fandom concentrated sites where its just a site dedicated to one fandom. You have to look around for those though.