android-app://com.google.android.googlequicksearchbox

What it is

com.google.android.googlequicksearchbox is the Android package name of the Google app. The android-app:// scheme is how Android identifies a native app as the source of a visit. Google's Discover feed also lives inside the Google app, but Google doesn't document which of the app's surfaces send this referrer.

Why there's no full URL

Apps aren't web pages, so there's no page URL to pass along. Android lets the app identify itself with an android-app:// URI built from its package name, and that's all the referrer contains.

What traffic it represents

Visits where someone tapped a link inside the Google Android app and the app (or the browser tab it opened) reported itself as the referrer.

Sources

Google - Apps on Google Play

Google - Google Play

it can also use the android-app: scheme to identify a native application that it came from.

Intent.EXTRA_REFERRER - Android Developers

otherwise, if known, an android-app: referrer URI containing the package name that started the Intent will be returned.

Activity.getReferrer() - Android Developers

You can set your app as the referrer when launching your Custom Tab. This way you can let websites know where their traffic is coming from.

Customizing the UI - Chrome Custom Tabs

Users get to their Discover experience in the Google app, on the Google.com mobile home page, and by swiping right from the homescreen on Pixel phones.

Search Console reporting for your site's Discover performance data