navigator.getInstalledRelatedApps()

...

Steps

1. Install app

   Sideload the companion app that declares a Digital Asset Link to this origin.

   ⬇ gira-related.apk

   adb install -r gira-related.apk

2. Launch website

   Open this page and press Re-check. It should report the play app.

   ↗ Open

3. Install PWA (resulting in WebAPK)

   Use Install PWA above (or menu → Add to Home screen), then re-open and Re-check.

What the manifest declares

"related_applications": [
  { "platform": "play",   "id":  "com.januschka.gira.related" },
  { "platform": "webapp", "url": "https://static.januschka.com/i-40915242/manifest.play.webmanifest" }
]