How do I get the Shortcuts app in iOS 12?
Shortcuts app is available for all the devices running iOS 12. You can use this App Store link to access the app.

Open this answer directly on your iPhone/iPad and tap on the link above to directly open the App Store page of the Shortcuts app and download it. Alternatively, open the link in Safari for macOS and AirDrop it to your iPhone/iPad. This will directly open the App Store page for the app where can download it.

Your devices aren't too old to run the app. There is no special device capabilities required for running the app. The app is supported on all the devices capable of running iOS 12. (For iPhones that goes as far back as iPhone 5s.)

Shortcuts used to be a 3rd party app called Workflow before Apple acquired the app and hired the development team. You can learn more about the app here.

Apple has shared a pretty in-depth user guide explaining how to use this big step forward for iOS - the features are baked deeply into iOS and 3rd party developers can write extensions that let shortcuts and Siri reach into those apps and do work and automation.


Shortcuts User Guide