My two main sources:
My local library website, it’s got a lot of user-curated lists that I’ll dig through if I read a book I like and want to expand on the genre/topic. If I see something interesting I’ll mark it ‘for later’ and check out when available.
StoryGraph, I enjoy perusing the reading lists of other users that share my taste (per the app). Reminds me of last.fm but for books.
Otherwise, sometimes I’ll simply search the subject matter and dig through posts on Reddit or other forums. I love a personal recommendation, I just don’t know a lot of people who read as much as I do (or share my taste).