>Does Apple have a method or system in place for developers to target older devices with the same app but using different builds within the Appstore?
Sort of...
In some examples, when an older download for an older iOS pre-exists in the store, the store will offer it to a user. This is not a scheme the dev can employ on their own, at will, however. The dev-facing fact is you can only place one build/version of the same app in the store at a time.
That said, Xcode allows the dev to build one app that is capable of supporting a range of iOS versions. Using the current v10.3 Xcode as an example, with iOS 12.x as the base or max iOS supported, the Deployment OS setting can be lowered to expand support, say for iOS 11.x
This of course means the features and code used in the app are compatible with those settings, the specifics of which vary depending on.
If you have additional questions/discussion, pls.start your own unique thread, rather that tagging someone else's, (especially one so old) so you can better track and close if solved, etc., linking to this this thread as needed, thanks and good luck.