Little of this, little of that.  I think better with pencil and paper.  I type faster than I write by hand, and my handwriting is awful.  Even I can't read it later sometimes. 

So hand-scrawled maps on whatever makes sense and college-ruled notebooks for sketching out ideas. Then 3-ring binder per campaign, with a combination of hand-written stuff and Word docs.  Also use Worldographer (Hexographer) for maps that will be used long enough to justify the extra effort (e.g. my local campaign area map).

I've also used 3x5 index cards, 4x6 index cards, and the backs of old business cards for various item/NPC notes.  But I find organizing them so tedious that I'd rather put the same information in documents, even if I do waste some paper reprinting eventually after changes.  Having the 3-ring binder in clear sections and good information in the footers of many Word docs instead of one big one helps a lot there. 

There will also be a smattering of Notepad, Excel docs, etc. for various pre-notes at items, but anything useful there tends to get moved to another document before it is kept long-term. 

I've looked at most of the online campaign tools but haven't found one yet that is even remotely close to what I would consider using.

I take notes on paper, and write stuff up later with vim and markdown.

I use the "Notes" app on my iphone.  The search function makes it easy to find anything, and I always have it with me, so I can jot down ideas as they come...and they are legible.

I use a huge mix. College-ruled notebooks, notecards, post-it notes, printed out maps, printed out monsters, and a wet-erase roll-up grid/hex map for at the table.

I use/have used word/excel, google sheets/docs, roll20, slack, obsidian portal, email, discord, whatsapp, Instagram messaging, and even dnd beyond for out of game communication and messaging. I insist that at the very least everyone use a messaging app, because this is the 21st century and I need a way for everyone to communicate without messages getting mixed up in texts and emails, which I use for family and professional life. I also don't want 20 text notifications if gamers are just sending orc dick jokes back and forth.

A friend of mine uses Kanka, which I find disorganized, and I hate their presentation. I think ObsidianPortal is obsolete. Dnd Beyond can only be used for 5th ed, which I want to stop playing, and it's terrible for integrating houserules and anything created by the dm. Slack came the closest to providing what I need, but I'm still looking for the perfect campaign manager, so I'm going to look up a lot the suggestions from this forum.

I tend to go all in on campaigns and campaign worlds whether I'm running it or not, but the commitment of the people I play with ends at the table. So a lot of the information is just for me, but it helps when I'm running the game.