Windows 10 mobile is running the same basic kernel as your pc. You can select if apps are allowed to run in the background in settings. In previous versions of the os, apps would be suspended and resumed. They are running all the time now. It is up to the app what it does when backgrounded. Some old apps will suspend themselves, other more modern ones won't. Edge runs well (too well) in the background. You can check what is running in developer mode by connecting to the built in web server. It looks a lot like task manager. You can see processes, and how much cpu they are using. Closing apps you are not using lowers the CPU use which in turn uses less battery. Windows 10 mobile is actually one of the most advanced os to ever run on a phone. It is not a cut down version like android or ios.