Aside from using various physical aids (Referee's Screens, equipment cards, a book of example tasks, and a physical notebook for temporary notes made at the game table), for my main notes I use OneNote ('cos I already had it, so why not?).
I have a dedicated OneNote workbook for gaming, which is organised at the top level using the PARA method (google it). So, for example, within Projects I have chapters for various games (including one for my current T5 campaign), while in Resources I have chapters for different game systems (including Traveller, obviously), and so on. (And within any chapter, OneNote allows you to have pages, sub-pages, and sub-sub-pages ... allowing for a degree of ad-hoc organising at the lower levels.)
I should also add that that OneNote workbook is on OneDrive (again, 'cos I already had it), which means I can access it both on my home PC, tablet PC *and* even from my cellphone if necessary. One of my players can't leave her home (reasons), so we play at her house ... hence the need for mobility (though I try to avoid screen use as much as possible during actual play).
There are other programs that offer similar features ... the key thing is how you organise your notes so you don't lose control and get swamped. (I speak from experience.)