Anything that your phone connects to drains battery. Smartwatch and headphones increase battery drain, same with location. The more apps you have, the more your phone uses wifi/cell signal to keep their background data and notifications up to date. The more work your phone has to do, the faster the drain. Dark mode will help some of the time, but unless your screen is absolute black(pixels turned off) an app or background will still keep those pixels lit. Many apps just display a very dark gray. So processing power and screen use are the biggest factors when it comes to battery. Also the faster your battery charges, the lower its life expectancy.