How exactly does an App Store developer add support for earlier versions of iOS?
It doesn't mess something up that you have newer version numbers that support older iOS targets than older version numbers. That's perfectly alright.

Remember that the Deployment Target is the minimum version of iOS that user's need to have.

The optimal way to do what you want is to set the deployment target to iOS 11, and then in your app conditionally use features for iOS 12 and iOS 13 depending on their availability.

This means that you can compile one single archive that contains a single app that will install on iOS 11, iOS 12 and iOS 13, and will offer different functionality depending on the iOS version on the user's device.

If you're using Swift for example, it could look something like this:

if #available(iOS 13, *) {
  // new stuff
} else {
  // old stuff
}