Most notes are written once and never opened again. That's not really a note-taking problem — it's a design problem. A note that's built to be reviewed later looks quite different from a note that's built to be taken quickly and forgotten.
A few habits make the difference:
Write for your future self, not your present self
When you're taking notes, the context is fresh in your head, so shorthand and half-finished thoughts make sense. Three weeks later, that same shorthand can be unreadable. Take an extra ten seconds per note to write it in a way that would make sense cold, with no memory of the lecture or reading it came from.
Separate the source from your own thinking
It helps to visually distinguish what a book or teacher said from what you thought about it — a question that occurred to you, a connection to something else you know, a place you got confused. Mixing these together makes notes read like a transcript; keeping them apart makes notes read like a conversation you can pick back up.
End each session with a short summary
After taking notes on a topic, spend two or three minutes writing a two- or three-sentence summary in your own words, without looking at what you just wrote. This does double duty: it's a quick active-recall check, and it gives future-you a fast way back into the material without rereading everything.
Review notes before they go cold
A quick pass over a set of notes within a day or two — cleaning up shorthand, adding a missing word, flagging something to look up — makes them far more usable weeks later than notes that are never touched again after the pen goes down.
None of this requires a particular app or notebook system. The format matters much less than whether the notes are written with a future reader in mind.