Write up whatever you feel you need to, an exam is a very different thing to prepare for. If you have a process for it then you do you .
Yeah plaintext is fine, I like using markdown but it’s more for function than keeping it pretty. If you do a lot of it maybe some kind of tagging system would help or at the very least some kind of organisation. There are some neat pieces of software around for this that you might want to look at.
What I meant was that I try and avoid writing up notes on topics I know are already better documented elsewhere or that I’ll use so often I’ll never have to refer back to them. For “things I know I’ll have to look back on” it’s normally a topic or piece of syntax that gets used very infrequently so it’s easy to forget or something I just could not wrap my head around at the time.
Perhaps the most recent thing I’ve tried to learn is SQL. Whilst I was starting out I had several pages of paper notes but only a small fraction of that was ever typed up. The practice and repetition meant certain things eventually went in.
Recorded notes are some brilliant examples of applications that were very useful but I’ll probably not need again any time soon. Quite a few projects are saved there and I’ll occasionally hunt down syntax from a project where I remember doing X but I can’t remember exactly how I did it. So there’s less on basic syntax and more on application (because the example includes syntax anyway and works better for me).