If you want to get statistics from the application usage and user tracking, you have to connect your Google Analytics and get the ID. We strongly recommend to create 2 application on Google analytics to differentiate iOS and Android users.
To get Application ID, login Google Analytics, on the top left, click and get the ID as described in the screenshot below (starting with "UA-"):
Then click on "Create App" to finish.
Wait a few hours for Google Analytics to ingest data.