the good old notepad,
That's my preference.
Is it like complicated things or simple ones?
It is both. I take very detailed and heavily linked notes (through my own PKMS based on index cards) and I also use notebooks for quick notes on the go notes. Have been doing so for years. BTW, there is a community related to Note-taking/Personal Knowledge Management: !pkms@lemmy.blahaj.zone
I don't use it to remember things (well, yes it helps with that too, but it's not the main purpose) I use it to help me think and create new ideas/associations between existing ideas, infos, whatever.
More info on 'my' note-taking system (it's not mine by any mean): https://en.wikipedia.org/wiki/Zettelkasten
Edit: published too early an unfinished post, sorry ;)