How to find what apps are running in the background of my iPhone 11

Apps don't really run in the background as once you move to something else, the app is suspended to permit you go back where you left off with the app. But to see the most recently used apps, with your finger, slide up from the bottom of your iPhone screen slowly towards the middle of the screen and pause. This will bring up the App Switcher of the apps you've used, but not fully closed. Once you see the apps in the middle of the screen, if you want to fully close any of them, use your finger to swipe the app up and it will be fully closed and no longer show on the screen.

As lobsterghost1 explained, apps aren't really running in the background. If what you're trying to accomplish is using less battery and less data, what you should probably be looking at is Background App Refresh. Go to Settings>General>Background App Refresh and turn it off for any apps that you don't need updating themselves in the background.