Get a Google Maps API key
Your application needs an API key to access the Google Maps servers. The type of key you need is a Key for Android applications. The key is free. You can use it with any of your applications that call the Google Maps Android API, and it supports an unlimited number of users.
Choose one of the following ways to get your API key:
The fast, easy way: Use the link provided in the google_maps_api.xml file that Android Studio created for you: Copy the link provided in the google_maps_api.xml file and paste it into your browser. The link takes you to the Google
Developers Console and supplies information via URL parameters, thus reducing the manual input required from you. Follow the instructions to create a new project on the console or select an existing project. Create an Android API key for your console project. Copy the resulting API key, go back to Android Studio, and paste the API key into the element in the google_maps_api.xml file.
I am wondering how do we find that file. Where is it located?
Go to res > values > google_maps_api.xml here you will find this