Do you want iOS to download and install future iOS updates automatically, or just download them in the background so you can install them when the time suits you? iOS 13.6 allows you to take control over this with a couple of taps.
As usual, finding where the settings are takes longer than making the changes.
These new settings live in a new entry in Settings > General > Software Update called Customize Automatic Updates. This replaces the previous Automatic Updates option.
Click on this and you are presented with options.
Here you have two options:
Download iOS Updates: This downloads new iOS updates over Wi-Fi and then allows you to choose a convenient time to install them.
Install iOS Updates: This installs downloaded updates automatically overnight, but your iPhone must be connected to Wi-Fi and charging for this to work.
If you're not the sort of person who needs their iPhone at night, you can have both options enabled and never again need to worry about updates as they will be applied for you as you sleep. If you need a bit more granularity, then the options are there.