Paper notebook. I wouldn’t recommend loose sheets of paper. :) After 15 years of writing notes on loose sheets I would start differently :)
Go Tim Ferris way - notebook where the first page is left for the table of contents, and number all even-numbered pages as first step.
My thoughts are so all over the place that I've settled on 3x5 note cards. It also makes the transition to the computer much easier, because I can re-arrange them in a way that is somewhat organized before taking a picture that gets transcribed
Possibility to re-arrange is ofc good when working on something in progress.
But how do you archive these cards? That always drove me mad so I use them only for something “encyclopaedical” otherwise it is too much messy.
You might want to check out Scrivener (Mac and Windows). I use that in lieu of note cards. The cork board function lets you re-arrange text and media to your hearts content and then compile the final output to Word, PDF whatever. It's designed for non-linear writing.
https://literatureandlatte.com
The point here is imho that you intentionally remove the computer from the path (in some part of your work) if you want to achieve some focused deep work (in case that you aren’t coding ofc :)