Showing a screen time statistic in addition to overall usage would be nice, but I'd like to know for how many hours my iPhone is actually running apps. When a PC goes to what it calls "sleep", it doesn't run any background tasks. Should be the same for an iPhone.
It can't do nothing when it sleeps - it still has to be a phone. Therefore it is always listening for phone calls, texts, push notifications, syncing services the user has set up, etc.
If you want it to do nothing, you need to turn it off, or at least turn off its connections.