For fiction and nonfiction, I annotate as I read (lately on kindle, but I mark up physical books too). This isn't like, brilliant or systematic annotation I just highlight stuff and add notes that can be anything from "lol" to "I wonder how this relates to..." to "this reminds me of..." I only look back at these notes if I am rereading the book or occasionally if I am looking for a quote.

Additionally, I have a note on my phone for whatever I'm currently reading. In between chapters, sections, or just at good stopping points I do pause and put in the main points from the chapter + my own thoughts. I find it more useful to do this after reading a section in my own words rather than taking notes as I go. I only revisit these notes if I take a long break from the book, in which case it is a useful refresher. For me doing it is more important than having it.

Finally, part of my wake-up routine in the morning is basically a memory game. While I'm getting ready for the day I try to reflect on the stuff I read or watched the day before and see if I can remember what it was about. If it's important - related to my job or just something I really want to learn - ill sometimes retread parts I can't recall or realize that I fully havent understood. With philosophy books for example I will sometimes think I have a handle on the content while reading and realize during my morning recall that I in fact do not.

Still, memory is a fragile thing and I'll generally forget a lot of the details of a book after a while away from it. I'm fine with that - in this case too I think doing is better than having. It's more important to me that I can deeply understand s book once than know it forever, and my recall method is in service of that.