I can see that iTunes and App Store has in Settings to not automatically download updates.
But what about iOS updates themselves? The recent iOS 11.3 update for example, is near 800MB, and when I use my main phone as a hotspot, then will my other iPhone silently download the iOS update?
The issue is important because the 800MB can cost up to US$12 at $15 per GB such as on Verizon, meaning that to download the update from iOS 11.2.6 to 11.3, it can cost me $12, and if 2 secondary phones, that means US$24 for downloading an update I wasn't aware of.