About the only time you need to actually manually close an app on the iPhone these days is if it is misbehaving.
How you force close an app depends on which iPhone you have.
If you have an iPhone 8 or earlier with a Touch ID button, double-tap the home button to activate the app switcher. Now swipe left/right to find the app you want to close, then swipe the app up.
For the newer iPhones with Face ID, swipe up from the bottom of the screen to the halfway point, and keep your finger on the screen to bring up the app switcher. Find the app you want to close and swipe it up to close it.