The same app on your home screen, full screen, with the wallet, your ghosts and your agents. It is not on the App Store or Google Play: you install it from the browser.
Scan with your phone's camera to open ghostdex.app/app.
The wallet lives on your phone only, encrypted with your passcode, and ghost keys are derived in the app from your signature. Ghostdex cannot see or recover either. Read getting started before you fund anything.