Curious to know how other GMs go about designing and filling out their campaigns with different material like items, NPCs, locations, etc. I've traditionally gone very deep and sometimes even granular (much to my own detriment because I bite off more than I can chew sometimes), and try to build out enough material to establish the setting and make everyone feel "unique" to the campaign setting.

What do you find works best for you? How deep do you like to go when building out the setting and materials? Are there any specific elements you focus more on as opposed to others?