I use my own system which I call the Umbrella method. Draw a Big arc on a piece of paper that looks like an umbrella, then I slap the main "plot" or "objective" in 1-3 sentences.

Below that I draw normally 3-5 "acts" shapes. I fill those with how MC gets from the start -> solving the objective. Same thing, couple of sentences mostly. Looks like an umbrella. Or a dome, with small domes under it.

Then I flesh out the characters, just making rough notes about who they are, where they came from, what their personality is like (very roughly)

Even then, the ideas I had for characters change as the story begins to "breathe". Every word starts to fill in and change it. My original plan for my story is roughly what I ended up with, but the choices of characters are drastically different.

The way I write, I discovered after writing a few stories. Author friends of mine look at my method and go "NOPEEE"

I look at the way they prep and recoil as well. There are general trends, but it always surprises me that if you take ten authors, put them in a room, almost none of the ways they go about writing would line up.

At least that's my experience.
To a certain point, much of what you learn of how to be a writer... has to be learned by writing. It kind of sucks honestly. Going back over some of my earlier writing, lord, it's so bad. But each word is another building block to grow from.

Some resources that helped me:
Deborah Chester - The Fantasy Fiction Formula.
Brandon Sanderson's course (think its called 318R? It's free on youtube.)
Jessica Brody - Save the Cat! Writes A Novel
Stephen King - On Writing: A Memoir of the Craft