How do I find out what apps are running in the background?

Then go Settings > Developer Options > Processes (or Settings > System > Developer Options > Running services.) Here you can view which processes are running, your used and available RAM, and which apps are using it up.

How do I keep apps from running in the background on Android?

Android – “App Run in Background Option”

1. Open the SETTINGS app. You will find the settings app on the home screen or apps tray.
2. Scroll down and click on DEVICE CARE.
3. Click on BATTERY options.
4. Click on APP POWER MANAGEMENT.
5. Click on PUT UNUSED APPS TO SLEEP in advanced settings.
6. Select the slider to OFF.