I think it is a mix of things - when i think of a story idea which is usually not when I am writing but in advance - i give it some structure in my head i.e a broad idea of the flow.
One other huge factor is I am a lazy writer ie i dont edit my stories - at all - so 90% of them are first drafts that are published. The only time i read a story after writing is to remove typos or basic errors that were fat finger issues and that is it.

That saves a ton of time - while probably also resulting in stories that arent the best out here but they work for my style.