[ if you take extensive notes ] -- any good solutions on finding/structuring/sorting notes?

I use google spreadsheets to store everything. Tables with components listings, card drafts, statistical tables, future artwork descriptions etc.
I tend to put everything into different tables and ability to include calculations right there on the spot is very useful.

It's usually one document per game. Different aspects of the game are on different pages of the document and those which became obsolete can be hidden, so that I can go back to them if needed.

It also makes it easy to share with other people if you are co-designing.

have run workshops for years with elements of that (but incomplete) - thanks for posting! Will definitely use.

Org-mode. Git to version-control and sync between my different computers/phones.

Org-mode (http://orgmode.org) that I mentioned about is all about keeping things organized and by default keeping as much as possible folded so you only see the parts of a large document that you need.

I mentioned in another thread recently my big document where I keep notes about almost 150 games. It is no problem at all navigating that file and never be bothered with any of the stuff I do not currently look at.
That is a reason I keep everything in one file, just make it simpler and faster to search and aggregate. When I get serious enough about a specific game I create its own folder for it somewhere (to store prototype graphics etc) and then it gets its own org-moder document (with cross-references so I can get from there to its entry in the big document and vice versa with a single click).

Works great for prototype rulebooks as well, since it has all the basic formatting needed as well as cross-references and being able to insert images. Exports to HTML and PDF and other formats. So I can do pretty much everything in org-mode until very far into development when I start to feel I need to convert it to something with more layout options (which, for simple print'n'play designs, sometimes is never).

Sections and subsections, sometimes cross-references, some tags and attached values for meta-data. The very basic features of org-mode, none of the advanced features. All just pure text and very quick to work with, everything keyboard-controlled, very quick to filter and search and navigate around.

First video I found on Google, goes through the basics, including way more stuff than I actually use day to day for tracking boardgame projects/designs/ideas.

https://www.youtube.com/watch?v=fgizHHd7nOo

Slower intro that actually explains what is going on:
https://www.youtube.com/watch?v=SzA2YODtgK4

I do pretty much the same as ElKobold - Google spreadsheets for mechanics and notes, including all game balancing calculations

Google Docs for Rulebook (and I make playtesting notes on a printed version - to be transferred later) - just because a printed version is easier to use whilst playtesting for me.

When I started playtesting I would do it from memory with a laptop in front of me. I found that that meant I relied too much on memory and missed things (changes and tweaks) I'd made to the rules.

Then I wrote a new draft of the rulebook and printed it out to be more useful - first mistake was not putting page numbers on it! When I found issues in the rules I would then turn to the relevent section and scribble in blue pen notes relating to those specific rules (so they stand out at a scan through)

At the end of a session I then update my google doc with the amends, and where a comment is more general or a query rather than a specific solution, I add it to page 1 of the rulebook (in the google doc) for future consideration.

I found it much easier/quicker to make amends on the printed rulebook itself than picking up a laptop, finding the right content and then editing (and making a note that this is just a suggestion not definite rule yet)

Equally I tried recording playtests but the time to play back was prohibitive for me - I know others like this approach and tbh - whatever works for you is my opinion.

It should be pointed out that EN has over 1 million paid subscribers, and has made well over 1 billion dollars, at best estimates. People are people and rarely run to praise anything, yet we tend to run out to bash things. Any online reviews are flawed because of this basic human nature. (Lots of people bash EN, but many more simply use it every single day and are perfectly happy with it.)
Personally, I don't care for EN, but I beta tested it and didn't care for it then either, for various reasons. It was once the new dog, then was seen as the industry leader, and now is trying to catch up. A lot of reviews for EN are also tainted by gossip about what happens at the company, or by a fundamental misunderstanding of what that , and related, technology is capable of.

I use a notebook. If it is important enough, I transcribe it to a computer.
I rarely have use for any cloud based systems since I am alone, don't travel much, and understand the underlying technology and dislike it.
Notebooks work well for me since I can easily flip pages this way and that to see what I did yesterday or last week. I can easily sketch ideas, flows, or anything else. I like that I can work in pencil, then go over what I like in pen, then go over that in marker for what I want to do & it gives me a little bit of a timeline & it is easy to mark why something doesn't work in a particular setting.

I tend to use ColorNote on my devices since it is designed to be just a notepad- offline, doesn't want me to use cloud, and works when I need it.