I use a combination of Goodnotes on my iPad to handwrite ideas, and then I get them into one or more "org" mode files in emacs for outlining, tagging, exporting, etc. (like obsidian but open source and more powerful but with a steeper learning curve)
I try to go as detailed as necessary. Which is more of an art than a science. Too detailed and you can't remember it or it's clunky to run. Too little, and you're stuck doing too much improv (yes that can be a thing when you mess up your skeleton of ideas).