To the scenario of one kid smacks the other (on the bum) and what does the (smacking) parent do -
I don't smack and obviously can't speak for everyone that does. But I get the impression that most people who do aren't using it for every little tiny everyday interaction, it's a "big guns" threat that they keep for persistent issues when necessary.
Or possibly like someone else said they don't use it consistently but it happens when they are at the end of their tether.
My next door neighbours used to smack their kids (though to be fair I never heard them actually do it) but I only ever really heard them threaten it, very occasionally, when the kids would not go to sleep at night - I was around a lot in the day and never heard them use it/the threat of it any other time. Generally the dad would shout the threat up the stairs and then one boy would defend the other and they would both go quiet - this was quick and consistent enough that I assume he must have followed through on the threat at least once. But I never heard it happen.
Not quite the same, but sometimes we talk to the DC in a stern/directive voice like "I told you to get changed, so get changed NOW" - not my preferred method honestly, but doesn't seem like a particularly unusual or controversial thing for a parent to do/say. And sometimes DC then use that same voice and directiveness back, like "Mummy!! I want you to play lego with me so you have to do that NOW!" and it sounds awful!! Every time it happens I'm torn three ways between: Stop being cheeky, children don't speak to adults like that / That's a horrible way to speak, nobody should speak to anyone like that / well duh, he's learned that's the proper way to get someone to comply with a request, we really need to lean more on other methods.
If he uses it at his brother then we generally say something like "You don't need to tell DS3, mummy/daddy will tell DS3, just concentrate on <whatever he's meant to be doing> thank you."