It depends on the type of nonfic I'm reading; I don't often take notes, but sometimes I'll snap photos of paragraphs that strike me as especially relevant - I did this several times while reading books about the 1918 Influenza pandemic (or I will annotate them on a re-read if it's a purchased/personal copy, like my Mary Roach books) - and text or share them with friends on social media.

For me, the best way to remember what I've read is to talk (or type!) about it with other people, either folks who've read the same book or friends I'm trying to convince to read it, because I connect my excitement about something cool/interesting/maddening I learnt from the book to that conversation.

Oh, and I read some nonfic at a much slower rate than I read fiction, so it may take me months-and-months to finish a "heavy" subject with a lot of technical language but I'll blow through a 400pp novel in like a day. I don't worry; I don't judge.