I am of uneven experience. For my screenplay, I have typed a couple hundred pages of various types: dialogue, storyboards, character backgrounds, story "fragments", "themes", "universe".
I had to tone down the abilities of my protagonists. Most movies want a more even balance between protagonists and antagonists.
My main techniques are 1. know my universe. 2. know the abilities of my chars. 3. know their secrets. 4. know their likes, dislikes and 5. my favorite -- their emotional reactions. I made my own attractive "list" outline system years ago when I was doing computer programming. I adapted it for lists of themes. I have over 20 "themes." (categories of topics). I have a wall covered with printouts for me to look at. Today, I will do a new print. I sorted the themes today, and moved out lesser ideas to a holding pond.
In the past, I didnt have enough ideas. Now, I have boatloads. I'm sure that most everyone will ask me to dial it back. HA ! We will see.