How do i keep my battery capacity healthy?

To learn about maximizing battery life and lifespan, read this support document --> Batteries - Maximizing Performance - Apple

If you want a charge to last longer, don't use energy intensive features such as videos. Turn off features such as GPS (of course if you lose your phone then you won't be able to locate it). It's all a matter of balancing charge life and features you want to use. Some of this is discussed in the article Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support

Connect your phone to the power adapter every night and leave it connected all night, every night.