How do I view apps running in background on my iPhone 11?

The only apps that are really running in the background are music or navigation apps. Go to Settings>General>Background App Refresh and you can see what other apps are allowed to update data in the background.

iOS dynamically manages memory without any user intervention.

If you're concerned about battery usage, turn off Background App Refresh for everything unless you really, really need it on. Also, go to Settings>Battery. You'll see suggestions on how to improve your battery life based on whats going on on your phone. And, you can see what apps are using the most battery life. You can also use Low Power Mode.

Glad it helped!