Does iOS download iOS updates when using a hotspot? 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.
In short, yes. This is how it works:


By default iOS does not allow the download of any single file above 100MB in size using your device's cellular/mobile data connection
When an iOS device is sharing its cellular/mobile data connection as a Personal Hotspot, that hotspot is seen as just another type of internet connection and not as a cellular/mobile data connection
More specifically, that connection will be seen as either a Wi-Fi interface, Bluetooth interface, or USB interface (depending on your personal hotspot settings)
Since the second device (i.e. the one connecting to the personal hotspot) is not seeing it as a cellular/mobile data connection, it can download files larger than the 100MB cap allowed on its own cellular/mobile data connection


There are a range of settings within iOS settings that you can use to manage what (incl. individual apps) have access to your cellular/mobile data connection, but these settings will not recognise a Personal Hotspot as anything other than another internet connection.