I hate that iOS downloads the updates in the background and uses up data quotas without warning and without consent from the user. The only solution to prevent inadvertently updating is to go to the storage settings and delete the downloaded update, only to have it get downloaded automatically again in a matter of days or weeks.
Low Data Mode in iOS 13 should make this stop (although they don't say it on the support page, it shouldn't do it if they are blocking App Store/iCloud updates...)
