Varies between species and body of water. But basically, fish are after the best food, security, and water temps.
Transition lines give you the option of hitting two features at once- like a sand-gravel, rock-mud, weed-sand, etc. Dropoffs offer a place for predators to slide up onto feeding flats from deeper security, or to corral baitfish that don't want to go above a "depth ceiling". The what/when/where is constantly changing, even to the hour of the day, but you can piece some general rules together- crappies love deep basins outside of spawning bays, perch love deep flats with bloodworms/crawfish, bluegill like weeds, etc. Basically- drill lots of holes in likely areas, then hop around to find fish (less effective in shallow water, where walking will spook fish). Fish finder is a huge advantage here.
Love the fact that you're not trying to look for everybody's honey hole. Good luck!