I really don't write notes for most books, because I remember most of it in my head and it slows me down a lot. But I've carried a notebook for a few years, I call it my "random thoughts journal", and I write things that pop in my head while reading there. Themes I find interesting, given phrases I like, a term I don't know or something to look into more

I don't have an organized style, per say. I just write bullet points, but I have a standard key I just use in my notebook anyways to keep track of everything: circular bullets for random thoughts, dashes for questions, squares for specific quotes/phrases/terms