Digital Assets Link link to my website

So for me, and maybe this is obvious to some, was the usage of the wrong sha key. What you need to do is go to your https://play.google.com/apps/publish/ console

Go to Release Management then to the App Signing page. Copy the SHA256 key from there.

Go to https://developers.google.com/digital-asset-links/tools/generator and enter the sha key, url and packagename there.

Upload this file to https://example.com/.well-known/assetlinks.json

Make sure that the file is in the correct location: A folder named .well-known.

Release your instant app

In your json file, the field package_name is set to com.kochchy.instantapptest.app.

However, the package name in your AndroidManifest.xml is set to com.kochchy.instantapptest.

They should match.

EDIT

Your structure looks pretty different from the one recommended by Google.

You shouldn't have to duplicate your code and resources. Instead, create a third module(let's call it base) to act as the base feature module, and move all your code and resources to there. Make sure its build.gradle includes these lines:

apply plugin: 'com.android.feature'

android {
    baseFeature true
    ...
}

dependencies {
    application project(':app')
    ...
}

In your app's build.gradle, make sure you have these lines in:

apply plugin: 'com.android.application'
...
dependencies {
    implementation project(':base')
}

Finally, in your instantapp's build.gradle:

apply plugin: 'com.android.instantapp'
...
dependencies {
    implementation project(':base')
}

You might need to make further changes, but this should be a good start. I strongly recommend you to take a look at this page, specially the section "Structure of a basic instant app".