For me it depends on the game and the format.
I tend to use handwritten notes in a notebook that I keep handy. I’ll then sometimes take those notes and type them into a word doc to make them more legible and organized. I try not to have more than 2 such pages so that I can easily reference them during play.
If I’m playing a game that requires enemy statblocks like D&D, then I usually copy them from electronic formats, minimize the entries, and paste as many as possible onto one page. If I don’t have an electronic version then I’ll just type up a minimal statblock and use that.
For less prep-heavy games, I usually just “refresh” and organize notes I’ve taken in play, and use those in the next session. I also use other resources when available. I have a google doc version of the Crews and Playbooks for Blades in the Dark. There’s a tab for the Crew and then one for each PC, and also one for Faction Statuses and Clocks that are in play. The beautiful thing about this is that the players update it as we play, and we can all view and edit live as needed.
Another example is when we’re playing remotely via Discord, I have a GM only channel that I can type notes into as needed. So I’ll do that during play, and then update and organize in between sessions.
For the face to face campaign of Spire that I just started, I mostly use handwritten notes. It’s a very player led game, so after the first session, I don’t expect to need a whole lot in the way of adventure prep. What I’ll need is NPC and Faction lists and descriptions and that kind of thing. I made a one page mind-map of the relevant factions and their relationships. I’ll mark notes on that during play, and then update it after play. All the possible NPC stats you need for Spire fit on about three printed pages, so I have them handy.
I just ordered a Campaign Planner Notebook from The Rook & The Raven, which I customized to be very suited to running Spire; it has lots of entries for factions and NPCs and the like. I’m looking forward to getting that.