As someone who has done writing, worldbuilding, tabletop DMing in multiple systems and a few other things.
When I was forced to write as an AuDHD kid in school, I didn't understand and wasn't able to cope with many of the requests for writing classes, namely the ones where you get given a topic or subject, keyword or phrase you had to theme something around.
To write about 'darkness' or 'travelling' or to include the theme of struggle and a whole list of abstract ideas.
My 'solution' at the time was to decide on characters who were consistent enough within limits, that no matter the topic, genre or context, I could fit them in as they were, and suddenly that is one less variable. If they were characters pulled from stories that existed or traits and thoughts I would put into a situation, that solved one of the key struggles that would freeze me due to too much information.
From there, I built up characters I was familiar with regardless of the setting, this allowed me, no matter the context, the tabletop genre, anything; to have something I didn't have to think about while exploring and learning the next thing.
Now I've gotten a lot better at writing characters and making cohesive groups and NPCs from over a decade of tabletop and other stuff things have somewhat changed. I can make consistent NPCs on the fly, now to reference and build off ideas I've set up years ago with no lag in-between them.
But as it stands, I will come up with a concept, an idea, sometimes with a few identity or traits attached, and then build out what seems logical for that story/draft.
After the basic idea, building out key details, I draft something, 1-2k words and that is effectively the 'brief' I'll re-read and build on when I decide I want to focus on the project. Usually noting every detail I feel is key to the topic/story, even the tone of the chapter or the way it is framed.
Sure I have weaponised my habits and mindset a bit, my AuDHD makes it extremely easy for me to have internal consistency across years worth of time gaps and other things, but I do trust my past ideas enough to let them go as they are.
Bonus struggles for having to write around the ARG I've kinda convinced myself to hide in the things I write.
Either way:
Base concept, A single line, ideal, character trait, expression, theme
Expand as needed for an opening draft/brief, something that you can use as footnotes for pitching an idea
Establish the main character further while developing the world and the conditions that needed to exist for the character to become that
Plan out the plot direction
Build the expectation of the MCs character arc
Start writing freestyle with the previous context assisting
It works both~. Though, the outcome may depend~.
