what exactly is this keystore.properties?
When you want to publish your app in the google play, you need to sign your app. When you create a signing configuration,Android Studio ,by default, adds your signing information in plain text to the module's build.gradle files. If you are working with a team or open-sourcing your code, you should move this sensitive information out of the build files so it is not easily accessible to others. To do this, you should create a separate properties file to store secure information and refer to that file in your build files. That file is the keystore properties file.
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Is it possible to create this file as it's not available in the github
repo files?
Create a file named keystore.properties in the root directory of your project. This file should contain your signing information, as follows:
storePassword=myStorePassword
keyPassword=mykeyPassword
keyAlias=myKeyAlias
storeFile=myStoreFileLocation
If you do not have store password, key password etc. you should sign your app.
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The whole information about this is in the Android Developer documentation, in the link I provided above.
keystore.properties
file is used to build a release apk, change your build variants to debug mode – Bot