If you want to analyze events and users across your business’s website, as well as its separate apps for iOS and Android — how should you set up your account?
Explanation:
When you’re setting up your properties, structure them in alignment with your user bases and business objectives. Because your app is on two different platforms, he needs two data streams — one for Android and one for iOS.
Remember, a property is the container for your reports based on the data you collect from your apps and sites. A data stream lives within a property and is a source of data from your app or website.
A flow of data from your website or app to Analytics. There are 3 types of data stream: Web (for websites), iOS (for iOS apps), and Android (for Android apps).
Read more here: https://support.google.com/analytics/answer/9355659