If you quit the app with the menu button then it is quit completely. Essentially, the "home" functionality works just like on iOS or Android. You're right that it is not always intuitive in situations when an app is hanging/frozen, but I'm not sure why you have to open other apps to get it to quit - I've never had that problem.
Disclosure: I used to work on the Xbox Live team at MSFT but no longer do.
That may be what's supposed to happen and what does in testing, but that's not real world behavior.
I had the YouTube app crash on my Xbox One just yesterday. I hit menu and selected quit. Waited a few seconds. Hit menu again and quit wasn't an available option which should mean that it's done being force quit. Started YouTube again and it will still hung playing audio with a frozen picture. Sometimes I'll start it again and it'll just show the YouTube logo for a few seconds then return to home. It'll often do this a few times in a row. I have maybe 6 people in my neighborhood with Xbox Ones that all experience the same issues.
Disclosure: I used to work on the Xbox Live team at MSFT but no longer do.