How much or what are the things you planned out before you start writing? Or does it change depending on what kind of stories you want to write? If so, what are the essentials that you definitely write down in all of them?
That depends on the types of stories I want to write. For my main psychological horror fantasy litrpg, other than a bit of planned out information after I finished my first arc to help support the next arc and inspire me, practically zero in terms of planning out. However, it has also been growing and changing and revising in my mind. Dreams, random bits of inspiration, sinking within the mire of depression, and trashed remnants of other stories have been consumed to make the story. So... I guess one could argue that it has the most preparation of any story I have made?
For a second story I have built (which has not been posted for public consumption), genuinely zero planning out. I got a premise in my mind, and it sprung onto paper easily (though the revisions were a pain and a half). It is a fanfic that drew on various psychopathic scientist tropes to build my character.
For the third (which has not been posted for public consumption either) (a transmigration-to-book horror superpowers slice-of-life comedy) it has required an incredible amount of preparation, with more than 30 personalities and characters needing to be constructed in advance, relationships planned in advance, world-building done prior, power system crowd sourced with the help of my friends, and more that don't come to mind.
Edit: Oh yeah. The less said about the amount of effort that is needed to be invested into the plot, the better. *Proceeds to talk about the plot* I have multiple side-plots, many easter eggs to place throughout the story, a whole cast of antagonists, themes that I will need to relay, deaths and betrayals that I must plan hundreds of chapters in advance... an apocalyptic/new-world creation ending potentially in the making... realistic friendships and a whole slice-of-life thing going on that I need to ensure it doesn't get too serious... a focus on comedy, which I suck at...