The screen is almost always the largest single consumer. Brightness, refresh rate and how long the display stays awake matter far more than which apps are installed, which is why a day of reading drains more than a day of messages.
Signal strength is the surprise second. When a phone sits in a weak-signal area, the radio increases transmit power and searches repeatedly, and that costs more energy than a strong connection ever does. A poor-coverage office can halve a day's battery.
Background activity comes third. Location updates, syncing and refresh happen quietly, and a single misbehaving app that keeps waking the phone will drain it overnight even while untouched.
Age matters too. Batteries lose capacity with each charge cycle, and a two-year-old phone may hold a fraction less charge while reporting the same percentage. The phone is not slower or faultier; the tank is simply smaller.